Firefox Mobile, also known as Fennec, is only truly available for the little-used (in the States) Maemo operating system, with alpha and pre-alpha versions available for Windows Mobile and Android, respectively. Basically, it's still going to be a while before you get the full Firefox experience on an iPhone or Android device. But the newly approved Firefox Home app for iOS (iPhone, iPad, and iPod Touch) is a pretty good halfway point.
Firefox Home is an add-on for the desktop version of Firefox (on Mac or Windows) that syncs with Mobile Safari on an iOS device. It's like opening up a portal to Firefox: You get access to your browsing history, bookmarks, and even the tabs you have open on your computer. You can browse all that stuff within the Firefox Home app, or open any site with Mobile Safari.
I wish it truly integrated with Mobile Safari rather than existing separately, but I suppose it's doubtful Apple would allow any developer, especially one like Firefox's creator, Mozilla, to modify Mobile Safari to that extent. And after the acceptance of Opera Mini into the App Store, it's pretty likely that Fennec will follow suit at some point, which would make Mozilla a challenger to Apple's mobile Web-browsing throne.
So it's a little bit clumsy, but still dead useful. I often leave home with directions or a map of my destination open in a tab on my PC, and it's a great idea to make it easy to check that stuff on the go. You can download Firefox Home from the App Store for free (link opens iTunes).