Mozilla Has No Plans to Port Firefox to iOS
Firefox and iOS users are out of luck, as Mozillas has no plans to port its popular browser to the Apple mobile operating system. The reason is a rather simple one, in that Apple forces all browsers to use WebKit. Mozilla uses its own Gecko HTML engine for rendering, and because of Apple's restriction there won't be an iOS port of Firefox. Mozilla use to have a Firefox Home iOS app that was focused on bookmarks and tab syncing, however it was pulled last September so the company could work on other projects. There was also an experimental browser shown in the middle of last year, but like every other iOS browser it was running WebKit. Until Apple loosens its restrictions on iOS browsers, don't expect to see Firefox on your iPhone, iPad, or iPod. There are other options, like Chrome, Opera, and iCab, if you want something other than Safari, but you're unable to set anything else as the default.
Source: Ars Technica