Firefox OS Simulator is a prototype add-on for emulating Mozilla’s upcoming operating system for mobile phones and currently it is at build 0.7. The main goal for version 1.0, which is in the works at the moment, is to offer developers a stable and easy to use testing ground for installing apps for Firefox OS.
Exceeding 65 MB in size, the add-on is a huge download and works on Firefox 16 and above. After installation (no browser restart is required), you can find it in the Web Developer section of the browser. Updates are installed automatically.
Because it actually runs B2G Desktop, Firefox OS Simulator runs smoothly and startup times are very fast.