The fullZoom feature of Firefox 3 is cool. Very cool. It's so cool that combined with the marvelous extensibility of Firefox, I think many extensions are going to use it and offer a wide set of new features based on it. Accessibility, shrink-fitting, mobile, the range of applications is large. But to allow such a set, we miss one important thing : when one piece of code or extension changes the fullzoom factor, the others need to be notified about it... That's why I filed bug 416226. It already works in my build, the patch is coming.
One minor nit : the DocumentViewer's fullzoom setter calls SetFullZoom on all DocumentViewer children. So if a document contains frames/iframes, the listener will receive a fullzoomchanged event per document in the window.
The other choice is a standalone listener system only for fullzoom but I really prefer a custom DOM event, it's cleaner and simpler. Please add opinions/suggestions to the bug itself, comments on the current blog article are closed on purpose.