nsIPrefBranch2 in Firefox 2.0

 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 preference which has changed

 @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 insures 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.

 @see nsIObserver
 @see removeObserver

 Remove a preference change observer.

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

 @see nsIObserver
 @see addObserver