Tab Effect is a Firefox extension that provides transitional effect (similar to Xgl) when switching the tabs (Check the screenshot below). Itís a cool idea and could open the door for the development of more 3D effects in the browser. This is version 1.0 and it was released yesterday. Itís a work in progress and It still has some bugs to be fixed. The comments left so far in the extension page will give you an idea of what to face when you install it. I hope the extension developer will fix those bugs soon.
(The cube effect shown above is within the Firefox browser, not any OS transitional effects).