New problem this, and it took some google-fu to find out a solution. I recently upgraded Iceweasel (the unbranded Firefox) on my Debian Jessie to the latest beta. I use xmonad as my windowmanager.
After the upgrade it was impossible to make HTML5 video go fullscreen. In fact F11 had stopped working as well. Iceweasel/Firefox became completely unresponsive and needed to be killed in order to make it work again. Super annoying.
Eventually I tracked down a solution on the Arch Linux wiki. The XMonad.Hooks.EwmhDesktops extension, which is found in the xmonad-contrib module resolves the problem.
You can just import it and then add an event hook to deal with the fullscreen event. Recompile xmonad and restart and you are good to go.
import XMonad.Hooks.EwmhDesktops
⋮
xmonad $ defaultConfig
{ modMask = mod4Mask
, handleEventHook = fullscreenEventHook
}