You need to post screenshot of the issue so others the verify the issues? And if this requires login that will be hard to know if this happens from login or no login to test the playback issue. So you need to try in Private Mode browsing to see if this isn't a Addon issue as well.

There is nothing to screenshot, just a video frame with a buffering circle.

I have no issue on the guardian site. see screenshot Since there were examples or links provide for the mirror, is this the site??? https://www.mirror.co/

Did you try Firefox in Safe Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode