To quote Apple's iOS dev guidelines:
"2.5.6 Apps that browse the web must use the appropriate WebKit framework and WebKit Javascript."
This is like telling people that a lexus isn't a real luxury car, it's just a re-skinned toyota. Most people don't care- they just want materials, tech, and nice seats. The skin is the car
Users dont care about the engine until they cant use some web based thing because apple has decided so.
It might not seem relevant on the surface, but the underlying technical differences could pose an issue for certain user cases, thus it's an important consideration. Not all Firefox addons work with the iOS version just like not every Toyota dealership carries parts for a Zupr4.
It does have some Mozilla added plugins and syncs with Firefox logins, but the meat of the application isn't really "Firefox".