This 2-line web page will hang Firefox 3 with Firebug on a Mac:
<body onload="debugger"> <img src="">
(With Firebug scripting enabled)
This completely hangs the Firefox user-interface, so you can’t access your other tabs. The only thing to do is a Force-Quit. I reported the bug but unfortunately I just don’t have the time and energy to learn how to fix it myself. Hoping somebody out there might have some useful insights.