The Jakkify extension does something similar by showing an attention catching soyjak on all thumbnails, thus equalizing them and it's funny.
Current behavior is that randomized timestamp thumbnails show up with just this extension affecting them.
Videos with user submitted thumbnails show up gray (no thumbnail at all).
Jakkify extension is fully nullified in both cases.
https://addons.mozilla.org/en-US/firefox/addon/youtube-jakkify/
The Jakkify extension does something similar by showing an attention catching soyjak on all thumbnails, thus equalizing them and it's funny. Current behavior is that randomized timestamp thumbnails show up with just this extension affecting them. Videos with user submitted thumbnails show up gray (no thumbnail at all). Jakkify extension is fully nullified in both cases. https://addons.mozilla.org/en-US/firefox/addon/youtube-jakkify/