In March, Firefox released a preview of the latest Firefox OS Simulator. Some developers may not have jumped on board for fear of it being an unstable preview build, but that fear is no longer valid as the stable build is now live.
Mozilla announced this week that Firefox OS Simulator version 3.0 is now available to all as a stable release. The original preview from March added features like push to device, rotation simulation, basic geolocation API simulation, manifest validation and more. The new stable build includes all of the above features, plus some new ones:
With this new release, Mozilla has also decided to revamp its Firefox OS Simulator documentation. You can take a look at that here. While you're at it, you should also check out the Firefox OS Simulator walkthrough example for some tips and tricks.
If you have yet to start developing for Firefox OS, you can grab the latest stable build of the Simulator at the Firefox add-ons Web site.