The Window Watcher component is supposed to be a repository for the topmost window. (Yes Window Mediator does this too, but Watcher also works in embedding clients.) Code that depends on this is starting to come online; it's time to hook it up.
The patch below hooks up the active window in Mozilla and the embedded webbrowser. It also makes WindowWatcher a little more robust and cleans up here and there.
r=chak [on the mfcembed part]
patch is in