As another user mentioned below, the problem gets somewhat better if you switch off of Firefox and come back, but it has persisted through Firefox 10 and into FIrefox 11. Even when it is acting "better" it still exhibits some strange behavior. I have attached a screen shot showing this.
I don't think it's a KDE problem, because it works perfectly with all other applications. QT/KDE apps work fine, GIMP, Pidgin, and other GTK apps work fine. WxWidgets-based apps (such as Filezilla) work just fine, and so does the LibreOffice. I have no idea exactly what is going on, nor how to describe it. Sometimes buttons get "stuck" or a menu randomly remains open. Often, clicking off of the menu does not cause it to go away.
Please let me know if there is anything that you would like me to try.
Status changed to 'Confirmed' because the bug affects multiple users.