To restore animated thumbnail previews, requires inline previews to be disabled in your YouTube settings. Note: not Greasemonkey compatible. v2.0 adds fallback option of a still image carousel for the homepage where YouTube has changed their data structure to no longer provide an_webp animated thumbnail urls, which is affecting some users; for such users non-homepage pages are unaffected and continue to use the an_webp animated thumbs.
Seems it stop working for me too :(