Runs absolutely fine in situations where battery isn't a concern (iMac)
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.
However, in terms of iOS, I am not certain how either of our experiences (both on macOS) would really be relevant at all to how Firefox would perform on iOS. As much as Apple has tried to make them similar lately, they are still different worlds.