Mozilla FireFox is a web-browser developed and published by Mozilla.

Issues

Vine videos (HTML5 videos) don't play

The issue presents itself with a static image appearing instead of a video playing. Vine videos especially have this problem. The problem appears to be at the fault of FireFox's built-in mp4 media player.

Solution

Disable the built-in mp4 media player:

Open a new tab
Type in about:config in the address bar
If confronted by a page saying "This might void your warranty!" click the "I'll be careful, I promise!" button
In the search bar at the top of the page copy and paste media.windows-media-foundation.enabled
Double-click on the media.windows-media-foundation.enabled string to disable it
Restart FireFox and try loading a Vine or HTML5 video