It is a weird effect that happens sometimes in gnome with firefox hit the
f11 button to go to fullscreen, then again to come back, this usually fixes
it. Taking the mouse pointer right to the top of the screen and then slowly
back down will sometimes kick it back in as well. The latter applies to other
windows apart from firefox. The other way is to turn off the sexy effects, ie
right click on desktop go to change desktop background, change desktop
effects and select none.