I’ve gone between a few but on desktop it’s Librewolf. Just like Firefox but more privacy by default. On Android I’ve tried Fennec, Brave, Iceraven, and Mull. Out of all of them Iceraven has been my favorite simply because of its support for desktop extensions.
You can use more extensions on other ones as well:
https://blog.mozilla.org/addons/2020/09/29/expanded-extension-support-in-firefox-for-android-nightly/
Works on Firefox Beta, Firefox Nightly, Fennec and Mull.