Firefox stops playing mp4 videos

Firefox since ever had h.264/mp4 support on my system. The browser stopped working with this format few days ago. Any reason for this?

Firefox uses gstreamer for this.

Check that you have gstreamer-plugins-bad-orig-addon and gstreamer-plugins-libav installed, and try to delete the gstreamer plugin cache (~/.cache/gstreamer-1.0/)

There were other threads about problems with gstreamer recently, that seems to have helped.
There is an issue at the moment, gstreamer-plugins-ugly fail to build and are therefore still at version 1.4.5, while the rest of gstreamer has been updated to 1.6.1.
But AFAICT the h.264/mp4 plugins are not part of gstreamer-plugins-ugly (and gstreamer-plugins-libav provides a fallback for mostly everything anyway), so it should still work.

in the meantime you can try and use ffmpeg directly for multimedia support it has worked for me
#1 disable gstreamer
in the address bar type about:config
set to false

enable ffmpeg

in the same preference window (about:config)
find and set to true all of these settings:

Could not find gstreamer-plugins-bad-orig-addon, so I used about:config which works for me.

It is available from Packman, which you need anyway for playing back mp4 videos. (unless you installed ffmpeg from the VLC repo e.g.)

But using ffmpeg directly is fine as well of course, if it works for you.