Today we have made Firefox OS 2.5 available worldwide. We are also making an early, experimental build of the OS – Firefox OS 2.5 Developer Preview – available for developers to download on Android devices.
So you can flash Firefox OS 2.5 as a standalone operating system, or run parts of it atop your existing Android device.
On a related note:
Firefox for iOS lets you take your favorite browser with you wherever you go with the Firefox features you already love including smart and flexible search, intuitive tab management, syncing with Firefox Accounts and Private Browsing.
iOS, of course, doesn’t provide real browser choice to its users, so even this Firefox iOS browser uses iOS’ own rendering engine.