Firefox OS comes with a suite of pre-installed applications called Gaia, which handle the fundamental functions of the phone such as settings, calls, SMS, taking and storing photos, etc.
For Web developers, the most important part to understand is that the entire user interface is a Web app, one that is capable of displaying and launching other Web apps. Any modifications you make to the user interface and any applications you create to run on Firefox OS are Web pages, albeit with enhanced access to the mobile device's hardware and services.
Firefox OS is currently under heavy development; we are constantly working on ways to make it easier for you to use and hack on Gaia and create apps. However, you need knowledge about systems in order to do things like build the entire Firefox OS stack, or flash a phone with a build of Firefox OS. To find such information, and lots more besides, head on over to our Firefox OS content zone.