This has been the case for like 20 years no? See https://en.wikipedia.org/wiki/GNU_IceCat#History

I remember using Iceweasel and other variations back in the day because otherwise they’d be infringing on Mozilla’s trademark. I don’t actually see anything wrong with this. You can still create a 1-1 clone/copy of Firefox and advertise it as such with a different name. But telling others it is Firefox would not be accurate as Mozilla has no control over it and others may introduce changes that negatively reflect on their brand.