To restore animated thumbnail previews, requires inline video previews to be disabled in your YouTube user settings. Note: not Greasemonkey compatible. v3 Add fix for new youtube update to sidebar vids; remove carousel fallback.
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Solved changing addon from Grasemonkey to Violentmonkey
Thanks for the comments regarding compatibility. I just did a test now and although the @match pattern issue can be resolved be having separate @match lines for http://... and https://... the script still isn't working in Greasemonkey. I was a user of Greasemonkey myself until recently when I switched to Violentmonkey; Greasemonkey hasn't had an update in 3 years so is lacking features others have and also, in my experience, there seems to be other compatibility issues with scripts trying to run @run-at document-start which this script requires. So for anyone else, the advice is this script is not Greasemonkey compatible and the recommendation is to migrate to Violentmonkey which is likewise open source but is actively maintained (existing scripts can be exported and imported between them in a few clicks).
Can't download the script because this message:
Ignoring @match pattern http*://www.youtube.com/* because: Error: @match: Invalid protocol (http*:) specified.
I'm using Greasemonkey. I hope you can solve the problem :)