I'm sure this is a duplicate bug, because I can't imagine that I'm the first to complain about this behaviour, but I can't find any bugs that specifically addresses this! I'm using Firefox 10.0b5 in Windows Vista, but I've seen the behaviour numerous times with different versions of both Firefox and Windows (not sure about other OS's). FYI, I'm not quite sure what the correct term is for what I'm calling the "browsing frame". I think it's pretty obvious, but if you're uncertain, it's the hbox node with the id "browser" (i.e., hbox#browser). =====Steps to Reproduce===== 1. Ensure browser.fullscreen.autohide=true and browser.fullscreen.animateUp=0 or 2. Enter full-screen mode via F11 or Menu -> Full Screen 3. Move cursor to top of screen to "show" toolbars. What happens: toolbars show up and force the browsing frame downward. Expected behaviour: toolbars should be an overlay on top of the browsing frame. For the correct implementation, try entering full-screen mode in IE (I have IE8, but it should work in all versions >= 7).