I use Fennec, which afaict is Android only. It's my understanding it removed various Mozilla analytics though some is said to remain. It's compiled independently from source by F-Droid, which supports reproducible builds.

Primary reasons I use it is for enabling about:config editing out of the box and third party addons (either Mozilla-approved ones like uBlock Origin or any arbitrary addons so long as they're in an addon 'collection', following the same procedure like Firefox Nightly until Mozilla fulfils their goal of easier addon support).