That's a developer POV. For users, Firefox is defined as the browser with that specific look and feel, that landing pages, those awesome plugins and marketplace, that tab sync, that password manager, etc.

You'd be hard-pressed to find a single non-developer who is even aware there is a difference in rendering engines (or javascript engines). The maximum they might know is that "this site is broken with that browser", but that won't relate to "rendering engine" in their mind.

So brining Firefox to iOS would give an awesome benefit for many users that love Firefox on the desktop, and encourage others for which the absence of Firefox on iOS is a nuisance or a blocker in its adoption.