I may be biased, but I like Lemmy Infinite Scroller. It started out as a 20 minute project just as a proof of concept and then evolved into me trying to make an infinite scroller that works really well. It works well, I can't recall if I made it open-source but I didn't obfuscate the code or anything like that so it's practically open-source anyway even if I didn't make a repo for it. I just published the manifest v3 update of it last week so it's pretty futureproof too.
Other obvious extensions are uBlock Origin, uMatrix, and surprisingly one called Dark Background and Light Text (though Dark Reader is also cool too).