iOS Firefox is not really Firefox. It's a Safari skin, because Apple doesn't like competition.
Firefox matters not because it's "not chromium" but because it's firefox, a non-profit dedicated to an open web.
(Note: I've been running Gecko based browsers almost since the beginning. I guess in those days it was probably easier to use a browser that isn't perfectly supported, but I'd probably do it again if it came to it. But I can recommend Firefox to whoever I want, since it works on every site except Linked In. I couldn't do that if it only worked well on one in two or three or four sites.)
https://en.wikipedia.org/wiki/Firefox_for_iOS
> Unlike Firefox on Android, Firefox for iOS does not support browser add-ons. Additionally, it uses Apple's Webkit rendering engine, rather than Mozilla's Gecko.