Firefox OS is an open source and free mobile OS concept by Mozilla based on the idea that the OS and apps can be build using open standards like HTML5 and JacaScript. It is currently under heavy development and there is a planned official commercial release in 2013.
Things you should know about Firefox OS:
Unlike Android, it’s truly open source from the ground up. Meaning everyone has access to all version all the time.
Initially, it’s targeted towards low-end devices. It runs pretty well with even 245mb Ram.
Third party apps can be written in HTML/js/CSS; so existing HTML5 apps can be easily ported or used on Firefox OS.
They already have an app marketplace.
You can try it out now using Firefox OS Simulator plugin. The Simulator is much faster than Android SDK Emulator.
Personal impression: The design and functionality are very much inspired by IOS and Android. Nothing original yet from UI standpoint.
Demo port has already been done on existing popular android devices. So there is a possibility that you can use it on your existing phone.
Expected release date 2013 in Brazil via Telefónica.