The number one issue with the browser ecosystem right now is Safari. iPhone browsers being forced to render with Safari's engine is already bad. Blocking browsers from having extensions is far worse in my opinion. Most of the reason I use Chrome and Firefox on my desktop is their rich array of extensions.
If I were Firefox, I'd sue Apple to hell for abusing their dominance in OS and device market share to block me from competing fairly in the browser market.
Even if every safari user switched (desktop and mobile), it still wouldn't do much more than double the marketshare and yet remain a small fraction of what Chrome has.
I find it difficult to say that iPhone safari is the reason that Firefox is having trouble when overall, iPhone safari has less of an overall market share than Firefox currently has.
If I were Firefox, I'd sue Apple to hell for abusing their dominance in OS and device market share to block me from competing fairly in the browser market.