@clarity
Librewolf, Waterfox, Icecat — all of these are based on FF, not sure if they are still maintained. But there is little point in it as they are basically Firefox with different defaults.
And it used to have a reusable core, but I don't think it's still maintained. Sadly, they have abandoned it and webapp/site-specific-browser mode too 😩
@clarity
Servo — their experimental engine in Rust (which modern Firefox is partially based on) is no longer maintained by Mozilla either, but Igalia took over it and now it passes the Acid2 test: https://www.phoronix.com/news/Servo-Passing-Acid2
Maybe one day it reaches the level of maturity enough for us to have browsers based on it.