Comment 6 for bug 320257

(Just realized that what I wrote in the previous post concerning the mouse pointer being located slightly to the right of the top-left corner is NOT true for firefox, therefore that statement should be ignored - although it shows that menus in firefox work differently that the other ones).
Anyway, I run a clean firefox (no extensions), the issue should therefore not be related to any of those.
I am also pretty certain that the issue is with the X drivers (as I can only notice the issue with the nvidia proprietary driver - haven't had the chance to test the one from ati though), but in particular I'd say it is an issue with how those drivers deal with the implementation of the menus in firefox. In fact, based on what I noticed above, I suspect that firefox menu's differ from the standard gtk menus in some way.