after going fullscreen and back on youtube taskbar disappears

After going fullscreen and back on youtube (or any other video player) firefox switches to some sort of "always on top" mode so when the windows taskbar is set to auto hide and I hover over it nothing happens. Can't access bottom right corner either to go to desktop. I only realised what actually happens by resizing firefox from fullscreen to see that the taskbar actually expands behind the firefox window. I hope that makes sense, anyway it's quite annoying, if anyone knows how to fix it please HELP!!