Mozilla’s Firefox OS is still in the very early alpha stages, but if you’d like to test your apps in the latest version of the Simulator, head on over to the download page (note that there are known issues running the simulator on Linux and Windows XP).

Firefox OS is Mozilla’s answer to the question how does Firefox stay relevant in an increasingly mobile world? Locked out of Apple’s iOS due to the platform’s developer limitations and only recently beginning to create a truly competitive browser on Android, Mozilla’s long term mobile plan is to create its own mobile operating system built entirely on open web technologies.

Although the company has since switched to the “Firefox OS” moniker, the original name, Boot2Gecko, neatly captures Mozilla’s take on the mobile operating system — essentially turning the Firefox web browser into an operating system.

Applications built for Firefox OS use nothing more than web development tools — everything is made with HTML, CSS and JavaScript — which then run atop Firefox’s Gecko rendering engine.

To make it possible to create full-featured mobile apps with only HTML and other web tools, Mozilla is relying heavily on device-level APIs to tap into everything from dialing phone numbers to listing contacts, taking photos and getting Wi-Fi information. Not all of the APIs Firefox OS uses are web standards yet, though Mozilla has submitted most of them to the W3C for consideration.

Mozilla hardly has a monopoly on using web tools to build mobile apps; that was Apple’s original plan for iOS and it’s also exactly what tools like Phonegap or Cordova allow you to do for iOS, Android and other mobile platforms. The difference with Firefox OS is that you don’t need to package your app up in a native container — there’s no need for Phonegap and its ilk.

While Firefox OS may use familiar web development technologies and may run on the same Gecko engine that already powers the Firefox web browser, developers still need a way to test their apps in a mobile environment, which is where the Firefox OS Simulator comes in.

To get started with the Simulator, first open up the “Simulator Manager” by selecting the new Firefox OS Simulator option in the Firefox Web Developer menu. In the Simulator Manager you’ll find controls to start and stop the Simulator and a JS Console option to see any error messages as you develop.

For more on how to get started using the Simulator and building apps for Firefox OS, see the Mozilla Hacks blog, especially the very thorough tutorial from Mozilla community member Luca Greco, who walks through nearly the entire process of building and testing an app on Firefox OS.