Mozilla would love to have Firefox on iOS. You just need to convince Apple to change their terms of service to allow real 3rd party browsers (as opposed to UI skins atop the slower version of Safari).
What chrome did is fine for me. I don't need a super snappy web browser for mobile sites. I'd be fine with a UI that syncs with Firefox on top of WebKit
I don't think Firefox would be keen on (1) having a webkit-based browser with the name Firefox and (2) having it be an intentionally-hobbled browser compared to Safari so Apple can ensure its browser is the 'fastest' on iOS. Google made the decision with Chrome, since it is also webkit, but it will likely bite them in the ass later as blink continues to be improved and webkit begins to stagnate more. And as they begin to have differing rendering bugs.