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Hi Fred

What have you tried so far  ?

Are you using a new profile for Firefox 78 ?
Are there any content blocking extensions installed and enabled ?

Anything relevant in the Web Console ?

I tried all the normal things. Haven't tried a new profile yet. See picture.

See the Chosen Solution (which also worked for others) in this thread:
https://support.mozilla.org/en-US/questions/1271112

CSS rules usually aren't important and can be hidden most of the time via the filter bar.
I notice that are are three hidden warnings.

Those thumbnail are WebP images, so make sure you allow this image format and that you do not (try to) force JPG.
See also the Network Monitor.

• "3-bar" menu button or Tools -> Web Developer
• https://developer.mozilla.org/en-US/Tools/Network_Monitor

Also see:

https://www.reddit.com/r/firefox/comments/dnjd4i/youtube_thumbnails_dont_load_anymore/

Still nothing.

I started in Safe Mode, and now the webpage is properly displayed. Did not work before.

I disabled all add-ons, and restarted adding each add-on one at a time. Added the last one, and no problem.

Go figure.

Could you let us know which add-on turned out to be the culprit ? Might be helpful for others with the same problem !

And the culprit was ......... (drumroll)

I disabled 'Use hardware acceleration when available' and no problem.