Unlikely. Opera Mini works by doing all the rendering on the server-side, and then sending the pre-rendered website to the client. It's not actually a browser, so much as an app that gets screenshots from a server-side browser.

Firefox, on the other hand, would be a real browser, running Javascript rendering and Gecko on the phone itself, which would not be allowed.

That said, I don't understand why anyone would want Firefox anyway. The iPhone's limited memory and CPU would likely make Firefox nearly unusable, while extensions would eat away at what little free space was left. Firefox isn't optimized for the mobile space, and I can't think of any features it would provide that would make it a better choice than Safari (or, on Android, the built-in WebKit browser).