Firefox provides one of two browser engines.
If there's only one browser engine the Web immediately closes up like a giant impenetrable clam. All of humanity will be left outside, doomed to scrape forlornly on the shell with our nails and dream of the pearls within.
Blink and WebKit have diverged enough that there are minimum 3 engines now, not 2.
Don't forget Ladybird. It's coming along nicely too and it's currently being rewritten in Rust, which should give it a leg up on the rest of the bunch.
There are a few others, mostly A.I. slop like NorthStar.