Bugzilla

Then it's possible that your computer lacks of libavcodec that Firefox uses to play H264 video. Because H264 is a proprietary codec, Firefox relies on the external libavcodec (eg. from FFmpeg) to decode H264. As Bryce mentioned in comment6, Chrome bundles that inside already because they paid the license fee. So you would need to install that first in order to play H264 video on Linux.

(In reply to Tim Coffey from comment #5)
> Wow! upgraded to Firefox 88 and still unable to play video. 
However, Firefox should show an info bar on the top to tell users about that they have to install libavcodec. (If your issue is lacking of libavcodec) If the info bar didn't show, then we might need to investigate that again.

Then it's possible that your computer lacks of libavcodec that Firefox uses to play H264 video. Because H264 is a proprietary codec, Firefox relies on the external libavcodec (eg. from FFmpeg) to decode H264. As Bryce mentioned in comment6, Chrome bundles that inside already because they paid the license fee. So you would need to install that first in order to play H264 video on Linux.

(In reply to Tim Coffey from comment #5)
> Wow! upgraded to Firefox 88 and still unable to play video. 

However, Firefox should show an info bar on the top to tell users about that they have to install libavcodec. (If your issue is lacking of libavcodec) If the info bar didn't show, then we might need to investigate that again.