nsIPrefBranch2 Interface

 Add a preference change observer. On preference changes, the following
 arguments will be passed to the nsIObserver.observe() method:
   aSubject - The nsIPrefBranch object (this)
   aTopic   - The string defined by NS_PREFBRANCH_PREFCHANGE_TOPIC_ID
   aData    - The name of the preference which has changed, relative to
              the |root| of the aSubject branch.

 aSubject.get*Pref(aData) will get the new value of the modified
 preference. For example, if your observer is registered with
 addObserver("bar.", ...) on a branch with root "foo.", modifying
 the preference "foo.bar.baz" will trigger the observer, and aData
 parameter will be "bar.baz".

 @param aDomain   The preference on which to listen for changes. This can
                  be the name of an entire branch to observe.
                  e.g. Holding the "root" prefbranch and calling
                  addObserver("foo.bar.", ...) will observe changes to
                  foo.bar.baz and foo.bar.bzip
 @param aObserver The object to be notified if the preference changes.
 @param aHoldWeak true  Hold a weak reference to |aObserver|. The object
                        must implement the nsISupportsWeakReference
                        interface or this will fail.
                  false Hold a strong reference to |aObserver|.

 @note
 Registering as a preference observer can open an object to potential
 cyclical references which will cause memory leaks. These cycles generally
 occur because an object both registers itself as an observer (causing the
 branch to hold a reference to the observer) and holds a reference to the
 branch object for the purpose of getting/setting preference values. There
 are 3 approaches which have been implemented in an attempt to avoid these
 situations.
 1) The nsPrefBranch object supports nsISupportsWeakReference. Any consumer
    may hold a weak reference to it instead of a strong one.
 2) The nsPrefBranch object listens for xpcom-shutdown and frees all of the
    objects currently in its observer list. This ensures that long lived
    objects (services for example) will be freed correctly.
 3) The observer can request to be held as a weak reference when it is
    registered. This insures that shorter lived objects (say one tied to an
    open window) will not fall into the cyclical reference trap.

 @note
 The list of registered observers may be changed during the dispatch of
 nsPref:changed notification. However, the observers are not guaranteed
 to be notified in any particular order, so you can't be sure whether the
 added/removed observer will be called during the notification when it
 is added/removed.

 @note
 It is possible to change preferences during the notification.

 @note
 It is not safe to change observers during this callback in Gecko 
 releases before 1.9. If you want a safe way to remove a pref observer,
 please use an nsITimer.

 @see nsIObserver
 @see removeObserver

void addObserver(in string aDomain, in nsIObserver aObserver, in boolean aHoldWeak)

 Remove a preference change observer.

 @param aDomain   The preference which is being observed for changes.
 @param aObserver An observer previously registered with addObserver().

 @note
 Note that you must call removeObserver() on the same nsIPrefBranch2
 instance on which you called addObserver() in order to remove aObserver;
 otherwise, the observer will not be removed.

 @see nsIObserver
 @see addObserver

void removeObserver(in string aDomain, in nsIObserver aObserver)