According to the bug Kjell L linked (and another bug linked from that bug), this is a bug in xmonad.
** Package changed: firefox (Ubuntu) => xmonad (Ubuntu) -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1561933 Title: Firefox doesn't play fullscreen in html5 videos To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/xmonad/+bug/1561933/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
