As mentioned in a previous post, Firefox 4 is suffering badly from a known bug in Compiz on Linux. After coming out of a screen saver, menus will disappear when hovered over.
Minimising and maximising the browser will fix the problem, but for a permanent fix, the only solution seems to be to disable Compix until a newer version of Compiz arrives in your distro. For Ubuntu users, that will be soon- Natty has the fix. For Debian users like me, Compiz will be disabled for quite a while.