Consider running Icecat. It's most convenient to use it by installing a fully libre distribution such as Parabola or Guix System. Distributions which respect the FSDG don't just exclude proprietary software, but actually take software like Firefox which is 99% libre, and chucks out the last 1% of junk, improving many defaults. You're unlikely to have ethical qualms with such an Icecat.

https://www.gnu.org/software/gnuzilla/ You can see how it's made here: https://git.savannah.gnu.org/cgit/gnuzilla.git/tree/makeicec...

Note that it looks like it disables sync, but that should still work, and you can add whatever addons you like.