This photo show the issue better

It's odd that the control bar on that video doesn't show any buffering like I would expect (compare attached). I wonder whether there is some issue with memory being full or some other transient explanation.

Otherwise, could you test in Firefox's Safe Mode? In Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement?

A user on Reddit reported that this was a more general Facebook issue and it's fixed now. Is it working for you again?

Yes, it started working again. What points to Firefox issue with Facebook is that it worked fione on Chrome all day. Just not on Firefox.

Okay. Apparently Firefox has some difference from Chrome that FB developers bumped into when modifying their video player. I don't think we have enough information to know how users can avoid that problem in the future (on FB or other sites). Hopefully, FB will submit a bug report to Mozilla with details so it can get fixed.