Is it a fork? I thought it was a heavily customized version of Firefox ESR with specific settings and defaults using the channels OEM configs. Looking at the binaries, it's definitely not the standard build anymore.
OEM configs have never been enough to implement everything they need in Tor Browser. They eventually started their uplift effort [1], to upstream all the patches and features they've added to it, so that they can possibly just use OEM configs.
Looking it up, I think the more accurate term is actually "partner repacks." [1] They're versions of Firefox shipped with different default settings and/or an extension included by default at install time, IIUC. Though reading this rather opaque page [2] it doesn't seem to explicitly preclude patches to the compiled code, though it seems like they would forbid that.
I haven't looked into it in a while, but last I knew you had to have MoFo sign-off on a bunch of stuff, including code changes (i.e., you're only allowed to use the Firefox branding if you get MoFo sign-off on those changes; you can always redistribute it without the trademarks).
