I use Ubuntu 12.04 with Firefox 19. If I watch a HTML5 video in fullscreen and I use keyboard shortcuts on my laptop (Asus N53SV-S1827V) e.g., shortcuts fn+{f10,f11,f12} (toggle mute, decrease/increase volume) or the separate volume buttons above my keyboard then the video will exit fullscreen mode (quite annoying when watching a video on YouTube as it will then automatically adjust the quality of the video and starts buffering, and does this once more if I change back to fullscreen). This doesn't happen when using Firefox on Windows 7. Neither does it happen when using Chrome on Ubuntu or when using flash to watch the same video (on YouTube). I was able to reproduce the problem with any HTML5 video (I tried it on four different websites), and it also happens on another laptop (Sony VAIO) with Ubuntu and Firefox. The problem thus seems to be with Firefox when using HTML5 on Ubuntu. Some more info on the keyboard shortcuts. fn+f9 (toggle touchpad) also results in the exiting of fullscreen as do fn+{up/down/left/right arrow} (media controls), but fn+{f5, f6, f7} (brightness up/down, turn off screen) or fn in combination with keys that don't have a fn function doesn't produce the problem. Also, using Firefox in safe mode doesn't solve the problem.