Firefox extensions are a good thing (since sliced bread and embedded ActiveX controls...), in fact some of them are my most compelling reasons for using and loving Firefox.
Anyway, so with that World Cup thing that's going on I installed an extension called FootieFox that tracks any goals being scored in real-time and pops up a small little notification message. All fine and dandy, great unobtrusive little widget. Well, right until "FootieFox Education Day", when the extension started showing a banner that overlaid everything else on the screen, blinding up in obnoxious slowness, sitting there for a few seconds, and blinding down painfully with no way to dismiss it except to wait.
I promptly uninstalled the extension. (Great extension, but that "I'm gonna block out everything else so you have to read me" banner thing annoyed me.)