A while ago I wrote about some annoying bugs that had affected me in Ubuntu, but not in Debian Lenny, and how they had "come back from the dead" in Debian Squeeze.
I've just worked out that two of the bugs (failure of copy and paste in Firefox and disappearing controls in Totem in full screen) were actually the same bug- a problem with Compiz described here. Another manifestation of the bug was in the "awesome bar". Typing a letter in Firefox's address bar usually brings up previously visited web sites beginning with or containing that letter- but on occasion this would not happen. All these unexpected behaviours result from the fact that in Compiz "somehow inactivity messes up the order of 'layers' to draw and stuff that is supposed to be on top is drawn below other windows" (Vaphell).
The bug has an even bigger impact on Firefox 4, with right-click menus and even the main menu disappearing when hovered over- a show stopper of a bug.
The head shot required to kill these bugs seems to be to disable Compiz.
The Firefox 4 Compiz bug is discussed on the Debian Forum, the Ubuntu forum, and the Linux Mint forum.