It doesn't, as I noted in another comment - "absolutely fine" overlooks notable issues with the macOS system experiences.

Furthermore, while this is a point to be debated... the thread here is not about Firefox on macOS, but on whether it'd run fine on iOS.

Given the performance of Firefox on Macbooks in general, I maintain my position that it would not fare well. The iMac has little to do with this. Given that they'll eventually need to have this running well on Apple Silicon, though, I'd be happy to eventually be wrong.