The application is called Firefox Home, and isn't so much a browser - it's more of a companion to your regular desktop Firefox installation. It hooks into said installation by copying your browser history, bookmarks, and the open tabs from your most recent browsing session onto your device. Add some Awesome Bar sauce on top, and you're done. Pages themselves are rendered by the WebKit engine that's part of the iPhone OS.
"Firefox Home for iPhone is part of a broader Mozilla effort to provide a more personal Web experience with more user control," writes Ragavan Srinivasan, product manager at Mozilla, "For devices or platforms where we're unable to provide the 'full' Firefox browser (either technically or due to policy), we aim to provide users with 'on the go' instant access to their personal Firefox history, bookmarks and open tabs on their iPhones, giving them another reason to keep loving Firefox on their desktops."
Mozilla plans to submit Firefox Home to the App Store in the coming three weeks, and from the looks of it, there appears to be little reason for Apple to reject it. It most certainly looks like a welcome mobile companion for Firefox users.