Mossop Status Update: 2010-07-30


  • Down to 2 blocking nominations in Toolkit
  • Completed beta3 patches, just awaiting review for one of them
  • Got review queue down to 2 patches
  • Produced a troubling graph of add-ons manager blockers:
  • Added support for installing up to 30 personas at a time
  • Work on the new details pane layout
  • Have patches in hand for 4 more blockers and patches in progress for about 10 others


  • Recovering from dental surgery
  • Get the new details pane ready for review
  • Get the new appearance pane ready for review
  • Look into whether I should just do all the styling while I'm already on the details pane


  • Need to talk with sdwilsh about download manager integration

How to extend the new Add-ons Manager (or how I built a simple greasemonkey clone in an evening)

One of the goals of the new add-ons manager API was to create something that was itself extensible. A couple of times in the past we’ve had to add new types of add-ons to the UI like Plugins and Personas. In both cases squeezing them into the UI was something of a kludge involving a bunch of custom code for each case. We already have a number of new types of add-ons that we want to add, things like search plugins which are currently managed by their own custom UI.

To help simplify this the API is largely type-agnostic. Internally it uses a number of so-called add-on “providers”, each of which may make information about add-ons available. Each provider is basically a JavaScript object with functions defined that the API can call to ask for information about add-ons that the provider knows about. The provider then just has to pass back JavaScript objects to represent each add-on. Each of these must have at minimum a set of required properties and functions and may also include a set of optional properties. The full set is defined in the API documentation.

With this design the user interface doesn’t need to care about implementation details of any of the providers, how they store their data or what exactly their add-ons are and do. Because each gives objects that obeys the same interface it can just display and manipulate them.

To try to show this all off I recently put together a small demo extension for the Mozilla Summit that registers a new type of add-on to be displayed in the main add-ons manager. This is a short overview of some of the highlights and I’ll make the code available for people to look at and take examples from. The add-on was a basic implementation of Greasemonkey allowing user scripts to be installed, managed through the add-ons manager and do it all as a restartless add-on.

Making a restartless add-on

Add-ons don’t have to be developed with Jetpack to make them restartless, although the Jetpack SDK certainly makes things easier on you, at the expense of less access to the internals of the platform.

The first thing to learn about making a restartless add-on is that you can forget about using XUL overlays or registering XPCOM components to be called at startup. Neither are supported at the moment, and maybe never will. Instead you have to provide a bootstrap script. This is a simple “bootstrap.js” file in the root of the extension that should include a “startup” and “shutdown” function. These are called whenever Firefox wants to start or stop your add-on either because the application is starting up or shutting down or the add-on is being enabled or disabled. You can also provide “install” and “uninstall” methods to be notified of those cases but that is probably unnecessary in most cases.

At startup the demo extension does some basic things. It registers for some observer notifications, registers a new add-on provider (I’ll talk more about that below) and does a little work to include itself in the add-ons manager UI (again, see below).

The rule is this. Anything your add-on does after being started must be undone by the shutdown function. The shutdown function often ends up being the inverse of startup, here it removes observer notification registrations, unregisters the add-on provider and removes itself from the UI. It also shuts down a database if it was opened.

Implementing a new provider

This extension implements probably the simplest possible provider. As far as the API goes all it supports is requesting a list of add-ons by type or a single add-on by ID. These functions pass add-on objects to the callbacks. For this add-on these objects are held in a database so that code does some fairy uninteresting (and horribly synchronous) sql queries and generates objects that the API expects.

Adding new types to the UI

Perhaps the hardest part of this extension is getting the new type of add-on to display in the UI. Unfortunately one thing that we haven’t implemented so far is any kind of auto-discovery of add-on types. Instead the UI works from a mostly hardcoded list. This is something that we think it would be nice to change but at the moment it seems unlikely that we wiull get time to before Firefox 4, unless someone wants to volunteer to do some of the work.

The demo extension works around this restriction by inserting some elements into the add-ons manager window whenever it detects it opening. In particular it adds an item to the category list with a value attribute “addons://list/user-script”. The add-ons manager UI uses this kind of custom URL to decide what to display when a category is selected. In this case it means displaying the normal list view (that plugins and extensions currently use) and to ask the API for add-ons of the type “user-script”. There is also some code there that overrides the normal string bundle that the manager uses to localize the text in the UI to allow adding in some additional strings. The code I am showing is of course badly written in that it is hardcoded and so could not be localized, please forgive me for cutting corners with the demo.

That is basically all that is needed to have the UI work to display the new add-ons from the registered provider however the demo also throws in some style rules to pretty things up with a custom icon

Notifying the UI of changes

When you implement your own provider you have to be sure to send out appropriate notifications whenever changes to the add-ons you manager happen so that any UI can update accordingly. I won’t go into too much detail here, hopefully the AddonListener and InstallListener API covers the events you need to know about enough. You can see the script database send out some of these notifications.

Get the full code

This has been a very short overview of the highlights of this demo, hopefully enough for the interested to pick up the code and make use of it themselves. The full source of the extension is available from the mercurial repository. Right now I wouldn’t really release this as an extension. As I’ve mentioned it uses synchronous sql queries (on every page load no less!) and cannot be localized. These things can be fixed but this was just made as a demo in basically one evening to show off the sorts of things that are possible with the new add-ons manager.

Introducing the new Add-ons Manager

Add-ons have really been an integral part of Firefox ever since before its first release. In fact Firefox has had an add-ons manager of some form since version 0.2 (which was at that time called Phoenix). Firefox 4 will include a completely redesigned add-ons manager and while many nightly testers will have already seen our work, now the beta release is out I wanted to talk about some of the new features it includes. If you’re interested I’m also writing a companion piece that talks about the history of the add-ons manager from its first appearance through to what the future may bring.

Add-ons Manager Redesign
The new Add-ons Manager

Changing the home of add-onsAdd-ons Manager in a tab

The most obvious change that you’ll see is that the add-ons manager in Firefox is no longer a separate window. Instead it appears as a tab within the main Firefox window, just like webpages. One of the big things that I think revolutionized web browsing was the introduction of tabs to allow you to keep many webpages open in the same window. This is because in general it is pretty difficult to keep track of multiple windows. Opening one tends to block the other making it difficult to quickly switch around them unless you are very careful about where you place them all. This really applies to parts of the user interface as well as webpages themselves. With the new design you’ll never go back to your webpage to read something and have a hard time finding the add-ons manager again.

The Firefox user experience team has been talking about putting parts of Firefox’s user interface into tabs for some time now and other web browsers have done this sort of thing already. When we were redesigning the add-ons manager this was one of the first choices that we made.

Giant robotLearn more about add-ons

One of the first big features that I worked on when I started at Mozilla was adding the “Get Add-ons” pane to the old add-ons manager. It was an area that allowed users investigating what this part of Firefox was for to see a list of a few recommended add-ons as well as do simple searching for add-ons listed at and install them right there without having to open a webpage. It turns out that many use this as their main way to get add-ons, somewhere around one in five of every add-on downloaded from comes through this pane. What we wanted to do with this redesign was to extend the sorts of information and recommendations that we can provide directly in the manager.

While still only showing a placeholder in the current beta the revamped Get Add-ons section will eventually include recommended and features add-ons, lists of the most popular add-ons as well as a short overview of what add-ons are for those who have never used them before.

Make changes without restarting

The new add-ons manager supports a new type of add-on, one that doesn’t need you to restart Firefox in order to use it. It is possible for quite a lot of extension developers to change their add-on to use this feature but perhaps the easiest way to support it is to use the new Jetpack SDK to build your add-ons. An example of an extension built on the SDK is MailPing, a simple tool to let you know wen you have new emails at your Google or Yahoo mail account.

Find the add-ons you need

Once you have a large number of add-ons installed it can be hard to find the one you are looking for if you want to make changes to it. In fact if you can’t remember whether the add-on you were looking for is a Plugin, Extension or Theme then the old manager made you look through each list till you found it. With the new manager we’ve added a few tools to help you. You can quickly sort the add-ons in a list in a few different ways. We’re also building search right in. Typing something in the search box will look through all your add-ons for you trying to guess which you were looking for. In the future this will also return results from the extensive catalogue of add-ons available from

See more about your add-ons

You’ll probably see that in the new manager there is room for more information about your add-ons that was previously unavailable unless you visited Although in the initial beta lots of this information is not real, by the time we release Firefox 4 we expect you to be able to see information about ratings and reviews (particularly important when looking for new add-ons to download) and have the ability to easily contribute to add-ons that you could not do without.

Still to come

This is all just the early betas so there is still a lot of new things to come. In particular the way the manager looks right now is the same on every platform and based on early designs. The user experience team are now finalizing how the manager should look on all platforms. Future betas will also bring better feedback for the update and install processes.

Hopefully you’ll find the new add-ons manager easier to use but either way we are always looking for feedback so why not let us know what you think?

History of the Add-ons Manager

With all of the work that has gone into the new add-ons manager for Firefox 4 I thought it would be interesting to take a quick look back at the history of this part of Firefox and a quick look at what the future may hold.

Phoenix 0.2

Even in the earliest versions of Firefox, extensions were supported using the old XPInstall style packages. These had some pretty fundamental problems though in that there was no built in support for uninstalling extensions nor any way to disable them. There wasn’t even an extension manager window to see what you had installed at first. The very first time that a list of extensions and themes appeared in Firefox was way back in version 0.2, back when the product was called Phoenix. It was a very basic user interface and appeared inside the preferences window.

Extensions in Phoenix 0.2
Extensions in Phoenix 0.2 (2002)

Firebird 0.6

After the product got a rename to Firebird the next incarnation of the manager split the themes and extensions into separate parts of the preferences window.

Extensions in Firebird 0.6
Extensions in Firebird 0.6 (2003)

Firefox 0.9

Everything changed when Firefox 0.9 came along with its standalone extension manager window and support for the new install.rdf packages which are essentially unchanged from the extension packages that are used today.

Extension Manager in Firefox 0.9
Extension Manager in Firefox 0.9 (2004)

Themes and extensions were displayed in different windows and a basic update service was in place, to be improved in the backend code for Firefox 1.0. This first version was mainly written by Ben Goodger

Firefox 1.5

Firefox 1.5 saw very few differences on the surface of the extension manager, some slight changes to the visual styling along with the rest of Firefox.

Extension Manager in Firefox 1.5
Extension Manager in Firefox 1.5 (2005)

Behind the scenes, Darin Fisher made large changes. He allowed the manager to support loading extensions from different locations on the system including the Windows registry. He also created the ability to just extract extensions into the profile folder, which would be detected automatically the next time Firefox ran. Rob Strong then took over ownership of the manager to get it stable for release. This also saw my first patch to the extension manager, the rather insignificant bug 307925.

Firefox 2.0

Firefox 2.0 finally combined the separate extensions and theme windows into the unified add-ons manager.

Add-ons Manager in Firefox 2.0
Add-ons Manager in Firefox 2.0 (2006)

Underneath more changes were going on including the first blocklist service to allow us to remotely disable extensions that were found to be harmful to users. Since its creation we have very rarely used this feature but when it has been used it has helped prevent security exploits and crashes.

Firefox 3.0

With Firefox 3 we started including Plugins in the add-ons manager window and for the first time you could download and install add-ons from directly within the add-ons manager. This was the first big feature that I worked on in Firefox and I’m always pretty happy when I hear from the add-ons team just how much it is used.

Add-ons Manager in Firefox 3.0
Add-ons Manager in Firefox 3.0 (2008)

Internally the blocklist got upgraded to support blocking plugins and new install locations were added for platforms other than Windows to support integration with other applications on the system.

Firefox 3.5 and 3.6

No real visual changes happened in Firefox 3.5 and 3.6 however Firefox 3.5 improved the blocklist service yet again, allowing for a couple of different levels of severity while Firefox 3.6 added support for warning users about outdated plugins on their system and brought the fledgling Personas into the manager allowing quick switching between simple backgrounds for the main Firefox window.

Firefox 4.0 beta

Firefox 4 is seeing a complete redesign of the add-ons manager adding support for extensions that don’t require restarts, an automatic update system with less interruptions to the user and a more useful way to discover new add-ons.

Add-ons Manager in Firefox 4 beta
Add-ons Manager in Firefox 4 beta (early 2010)

Behind the scenes the new add-ons manager is now capable of managing new types of add-ons more easily than it was possible before.

Firefox 4.0

The user experience team are hard at work making the final designs for how the add-ons manager will look in the final Firefox 4 release. While only preliminary this is a quick idea of the sort of thing that they are going for:

Potential design for Firefox 4 final
Potential design for Firefox 4 final (maybe late 2010)

In the future we have plans to bring more types of add-ons into the main manager. Things like search plugins which currently are managed by their own custom window can fit in here. We also want to simplify customizing your extensions. Although it is unlikely we would stop allowing extension developers to create their own preferences windows we are looking into adding support for changing simple settings directly in the add-ons manager rather than needing to open new window. We’re hopeful that we can set up more automated ways of updating your installed plugins which are often the cause of security and stability problems and we want to significantly improve theme selection to make it easier to see what themes are available and switch between them.

It’s possible that some of these things may even happen in time for Firefox 4.0 but time is running short to get new things in before the final release.

Mossop Status Update: 2010-07-02


  • Landed the new notifications for add-ons installs
  • Triaged blockers for beta2
  • Fixed a large stack of blockers including all the known DB schema changes
  • Worked on a nice demo for the new add-ons manager's capabilities
  • Finished reviewing the component registration bits


  • Write some blog posts ready for the beta release
  • Fix some issues that Unfocused needs for his update work
  • Look into some of the intermittent timeouts that are happening in tests