Comment # 10 on bug 1171702 from Jon Brightwell

Sorry, this one is working as intended. I spent quite a bit of time on it until
I realised they were cheating. The front page of the [GIF] button is actually
GIFs. Everything below that is really video. You can see this by doing
ctrl+shift+i and inspecting the elements.

Basically anything h.264 or 265 encoded will not play in the version from the
official repo until those patents expire.

Real GIFs work fine. Anything using vp9 or mpeg2 or webm also works fine
(including most youtube links).

If they're important to you, use the tar, flatpak or wait for the patent
encumbered version from packman that Mr Galli is working on.