Firefox for Android is a great mobile browser, but it's not exactly popular. Mozilla could make it more popular if it were to release a version of its browser on iOS, but that's not going to happen anytime soon.
Mozilla's vice president of products, Jay Sullivan, was on a panel at SXSW talking about the mobile browser wars. On the topic of iOS, he said that Mozilla has no plans to bring Firefox to the platform. He also said that you won't see Firefox on iOS as long as Apple continues to restrict third-party browsers.
The main issue here is that Mozilla can't use its Gecko rendering engine on iOS. Apple requires all browsers to use the WebKit rendering engine that's employed by Safari, Google Chrome and many others. It doesn't help that Safari is the default Web browser in iOS, and users can't change that unless they jailbreak their phone.
Mozilla could easily get on iOS if it switched to WebKit, but the company doesn't appear to be doing that anytime soon. Instead, it will push for Apple, and others, to allow more browser choice on platforms. It's this choice that makes the Web great according to Sullivan.
Besides, Mozilla can't focus on iOS right now as it's currently busy with getting Firefox OS out to market. The new mobile OS poses no threat to Apple, but it's definitely doing things the Mozilla way - fiercely independent and maybe even a little reckless.[h/t: CNET]