Please provide a specific link and your device. I am able to play http://people.mozilla.org/~cpeterson/videos/toodee-480p.mp4 on a nexus 5 4.4.3.

I am using Samsung Note 2. Looks like a issue with samsung phones. Try to play Any video from cnn.com. The same happens with note 3 with 4.4.3 also.

I have the same problem on my Samsung Galaxy Note 2. Firefox will not play most videos on facebook.com, which it used to do just fine. The problem started a little over one month ago, I think. Other browsers on the phone have no problem playing videos on facebook.com.

Error message is a gray box on a gray screen with a light gray X that says "Video can't be played because the file is corrupt".

I have done a hard reset on my phone, wiping all personal data and apps, and reloaded just Firefox for Android from the Play Store, and the problem remains. I really prefer Firefox, but can't use it now.

We do a lot playback of mp4 movies locally on Samsung Galaxy Tab's and Note's smartphones. With Firefox v31.0 we have a lot of jerky / choppy playback of mp4's. Earlier it was no problem. I think there is something changed in Firefox. Please guys can you look at it?

When we play movies with the standard videoplayer on Android 4.4.2 of 4.4.3 movies runs fine. So problem must be Firefox.