I can easily replicate the issue on any system. 1.) Launch Firefox. 2.) Open several tabs of varying content making sure at least 1 is a flash video. 3.) tear off tab that is playing flash video, and send to second screen. 4.) Try to enable full screen playback in the new tab. If the video is playing, the playback controls will not show up. If you pause it you will get the playback controls but when you enable full screen it will go to the other monitor and then disappear behind the main Firefox instance. (Every once in a while it will stay up front but it's still on the wrong screen) If you reattach the tab into the main Firefox instance it will behave normally again. Firefox 28 (No Add ons) Windows 8.1 x64 (Although I've had the same problem in other OS's)