There used to be some work on getting Gecko on iOS. Since Apple does not allow rendering enginges alongside WebKit in iOS apps, the work was abandoned. It is now revived, according to
https://blog.mozilla.org/meeting-notes/archives/2084#ted – it is unclear, however, if this is a personal project or something really taken into consideration by Mozilla.
Mozilla has previously announced to build a browser for iOS, which is built on top of WebKit, though.