One of the problems with writing a restartless add-on is that you have to be careful about undoing anything your add-on does when it is told to shutdown. This means that right now some features of the platform can’t be used as we have no way to undo them. Recently I made this list a little shorter by making it possible to unload JS modules loaded with
Components.utils.unload(uri) and the module loaded from that URI will be unloaded. Take care when you do this because something might still have references into it which will stop working. Firefox also caches the module’s code for fast loading the next time around. The add-ons manager clears this cache when your add-on is updated or uninstalled so you mostly don’t have to worry about it but if you do something strange like unload a module, manually alter the file and then import it again you won’t get the latest code.
This support is in the upcoming Aurora build.