nsIThreadObserver in Firefox 3.0

 This method is called (from nsIThread::ProcessNextEvent) after an event
 is processed.  This method is only called on the target thread.

 @param thread
   The thread that processed another event.
 @param recursionDepth
   Indicates the number of calls to ProcessNextEvent on the call stack in
   addition to the current call.

 This interface provides the observer with hooks to implement a layered
 event queue.  For example, it is possible to overlay processing events
 for a GUI toolkit on top of the events for a thread:

   var NativeQueue;
   Observer = {
     onDispatchedEvent(thread) {
       NativeQueue.signal();
     }
     onProcessNextEvent(thread, mayWait, recursionDepth) {
       if (NativeQueue.hasNextEvent())
         NativeQueue.processNextEvent();
       while (mayWait && !thread.hasPendingEvent()) {
         NativeQueue.wait();
         NativeQueue.processNextEvent();
       }
     }
   };

 NOTE: The implementation of this interface must be threadsafe.
 
 NOTE: It is valid to change the thread's observer during a call to an
       observer method.


 This method is called after an event has been dispatched to the thread.
 This method may be called from any thread. 

 @param thread
   The thread where the event is being dispatched.

 This method is called (from nsIThread::ProcessNextEvent) before an event
 is processed.  This method is only called on the target thread.

 @param thread
   The thread being asked to process another event.
 @param mayWait
   Indicates whether or not the method is allowed to block the calling
   thread.  For example, this parameter is false during thread shutdown.
 @param recursionDepth
   Indicates the number of calls to ProcessNextEvent on the call stack in
   addition to the current call.