According to Mozilla's Vice President Jay Sullivan, Apple's iOS won't be seeing a Firefox web browser app. The reason for this is easily understood: Apple won't allow an app to be set as the default web browser. This means that anytime a person clicks a link in an e-mail, Safari is opened by default.
Mozilla used to have a Firefox for iOS app up until it pulled the app in September of 2012. Sullivan says that Mozilla may change its stance, but only if Apple were to play fair and allow third-party web browsers to be set as the default.
Besides, Firefox doesn't really need Apple. Mozilla is currently working on Firefox OS, which, if it catches on, could easily make up for the small market share forgone by not dealing with Apple. Additionally, any other browsers won't benefit much by having an iOS version as they suffer from the same default browser issue.