Try disabling graphics hardware acceleration in Firefox. Since this feature was added to Firefox it has gradually improved but there are still a few glitches.

You will need to restart Firefox for this to take effect so save all work first (e.g., mail you are composing, online documents you're editing, etc.,) and then perform these steps:

In Firefox 54 and below:

   Click the menu button New Fx Menu and select Options (Windows) or Preferences (Mac, Linux).
   Select the Advanced panel and the General tab.
   Uncheck Use hardware acceleration when available.
   Close Firefox completely and then restart Firefox to see if the problem persists. 

In Firefox 55 and above:

   Click the menu button New Fx Menu and select Options (Windows) or Preferences (Mac, Linux).
   Select the General panel.
   Under Performance, uncheck Use recommended performance settings. Additional settings will be displayed.

   Fx55Performance-disableHWA

   Uncheck Use hardware acceleration when available.
   Close Firefox completely and then restart Firefox to see if the problem persists. 

Did this fix your problems? Please report back to us!

If the problem is resolved, you should check for updates for your graphics driver by following the steps mentioned in these Knowledge base articles:

extensions, themes and hardware acceleration issues to solve common Firefox problems click here

   your graphics drivers to use hardware acceleration and WebGL Upgrade your graphics drivers to use hardware acceleration and WebGL 

Thank you.

On Windows, Firefox requires Media Foundation to decode MPEG videos such as MP4/H.264. Microsoft never released Media Foundation for XP, only for Vista and later. So on Windows XP, Firefox cannot play MP4 videos in its built-in player. Mozilla is not doing any more development for XP, so this is very unlikely to change.

There was a hack involving the Adobe Primetime plugin, but after all these years, I have no idea whether it still works. If you still have that plugin installed, you could review old threads like the following for more info:

https://support.mozilla.org/questions/1235544

You can try if the solution posted here to add support for playing MP4 files works for you.

• http://www.msfn.org/board/topic/175591-enable-mp4-h264-aac-html5-video-in-firefox-on-windows-xp-without-flash/