A decent chunk of the users who bothered installing an adblock would also be bothered enough to install a FF fork with adblock, so I doubt the revenue increase would be much.
As for calling it "off-mission": yes, what's even the point of FF if that's the route it goes on?
Do any of these forks have the ability to sync, either with Firefox or something self hosted? Or are they all just basically reskins with a single toggle added or such?
Yes you can actually self-host both Firefox sync server [1] and use Firefox accounts (which also can be self-hosted [2] and someone put something simpler in docker image [3]). And those can be used even with Firefox itself not only the forks.
Good point. I'd actually be happy to pay a couple of bucks a month for a good syncing solution based on an open source protocol, to make it easier for me to use the same history and preferences across browsers, IDEs and other such tools. It's actually a similar need and setup to that of a password manager, so I wonder if this is something Bitwarden could take on.
As for calling it "off-mission": yes, what's even the point of FF if that's the route it goes on?