There are different reasons to support alternative browsers. While Firefox on IOS definitely does ad and tracker blocking (which is great!) Some people are concerned that the underlying engine is restricted on IOS to webkit. So it may very well be a better experience for you than safari, but that alone does not satisfy concerns of homogenization in the browser space on Apple devices.
Yes it’s WebKit underneath but the “skin” itself is crap. They may not be able to have extensions on it but at least they could fix the UI and add ad blocking and tracking protection to it.