Found this notification this morning on my pixel 6.
- Fennec - Firefox build with some proprietary stiff removed; repo
- IronFox - Firefox fork (forked from Mull) with a bunch of hardening changes (notably
resistFingerprintingenabled); repo
IronFox is more ambitious, which means higher maintenance load and more likely to fall behind. Fennec is much simpler, so less likely to fall behind, but also doesn't change much from Firefox.