Kiwi Browser and Yandex Browser lets you use any Chromium extension, and Firefox Nightly, with some hacks, lets you use any Firefox addon available on the webstore.
Fennec on F-Droid is the best for not having to resort to beta or nightly builds. However, none of the Firefox options let you sideload your own extensions like Kiwi.
Kiwi Browser and Yandex Browser lets you use any Chromium extension, and Firefox Nightly, with some hacks, lets you use any Firefox addon available on the webstore.
Firefox’s stable version also comes with uBlock.
Do not use Kiwi Browser. It’s based on an extremely out of date version of chromium and therefore very vulnerable.
The hacky extension workaround also works for Firefox Beta now if you don’t want to deal with as many glitches that Nightly might have.
Fennec on F-Droid is the best for not having to resort to beta or nightly builds. However, none of the Firefox options let you sideload your own extensions like Kiwi.
Okay but I don’t want to use Chromium.
Fair enough! Definitely give Fennec a try. I switch between it and Kiwi myself (the latter for a few paywall extensions and PWA).