Tested with Firefox's ubuntu ppa 32.0a1 (2014-05-08): work
Tested with Firefox's Mozilla 30.0 beta: fail, mp4 can't be played
Firefox' ubuntu ppa should have some modifications: one side mp4 could be launched in ppa's version on other side ppa's version could not use F10 to acess to menu bar like in Mozilla's version
Tested with Firefox's ubuntu ppa 32.0a1 (2014-05-08): work
Tested with Firefox's Mozilla 30.0 beta: fail, mp4 can't be played
Firefox' ubuntu ppa should have some modifications: one side mp4 could be launched in ppa's version on other side ppa's version could not use F10 to acess to menu bar like in Mozilla's version