I would expect a corrupt toolbars, but it could be an extension problem. The first item will reset your toolbars, so suggest you take a picture of them first so you can customize them as you had them before.

• Corrupt localstore.rdf - MozillaZine Knowledge Base
• Toolbar customization - MozillaZine Knowledge Base
• Problematic extensions - MozillaZine Knowledge Base

I reset the toolbars and the problem persists. Actually, I reset all my settings and that didn't help. Now, I dont seem to experience the problem when I disable all my add ons. I went though one by one and disabled one add on at a time and the problem persisted, so I can't find one single add on to blame. Apparently there is a conflict between more than one of the add ons i use and FF4.

I have the exact same problem. I've tried resetting my toolbar (didn't help) but I haven't yet tried disabling my extensions. What's interesting is that in my case, this only happens when firefox is trying to load a page. If it's a page with a large number of elements and I try to click on, say, a folder in my bookmarks toolbar and select one of the bookmarks it contains, the folder repeatedly loses focus and closes over and over again until the page I'm currently on has finished loading. Another point of interest is that I've only noticed this on my netbook. So in addition to one or more problematic extensions, I wonder if performance might be a factor too.