Asides from the kinda-shady crypto stuff and the other things that've already been mentioned, just philosophically it should be kinda evident that over-concentration on one corporate controlled rendering engine isn't a good thing. Google wants the internet to be a walled garden with themselves as the sole decision makers so they can stuff ads down your throat.
Gecko's web compat is bad largely because of this over-concentration.
That's not the only point though. IIRC, they also remove telemetry, and pocket as well as some other things. I personally turn back on persistent sessions and history, but leave all the other privacy features there.