Please ignore helpfox102's post and don't call that number   -   it's a scam !

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

If there is still a problem, Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Did this help?

While you are in safe mode;

Try disabling graphics hardware acceleration in Firefox. Since this feature was added to Firefox it has gradually improved but there are still a few glitches.

How to disable Hardware Acceleration {web link}

I try all the options and I'm still having the same problem. I have this working on my android phone on Chrome, and I had this working on Microsoft Edge as well.

Make sure you have the latest FFmpeg and libavcodec packages installed.

FFmpeg was the problem, I didn't have FFmpeg installed on my Linux machine. Thanks cor-el.

ffmpeg was my problem as well. Thanks for your help.

Same, installing FFmpeg via the Software Manager, and restarting Firefox fixed the issue on my Linux Mint 20.

Same, I installing FFmpeg via https://software.opensuse.org/package/ffmpeg on my opensuse leap 15.2, restarting firefox and it's works