Firefox OS apps are nothing more than Open Web apps, that is HTML5 Apps, that are installed on a phone running Firefox OS. This means that you can directly reuse the knowledge that you have about front-end development to create awesome applications running on Firefox OS phones.
To learn more about the techniques involved in making apps rather than Web sites, study our tutorial: Getting started with making apps. Developing an app for a specific OS has a few caveats. You'll get the relevant information in our quick guide: Writing a Web app for Firefox OS.
Android;clause. For example:
Mozilla/5.0 (Mobile; rv:18.0) Gecko/18.0 Firefox/18.0.