FirefoxOS development is really easy if you have experience with front-end web development. Apps are very similar to chrome extensions, with .manifest files set up to control the application, and html / css to handle the content and appearance.
With this technology, it’s pretty easy to shoehorn a webpage into an app. So if created an HTML app using canvas, as I have done with my Protect Your Thing game, you can use something like the FirefoxOS boilerplate app to get yourself up and running quickly. Adding your app to the marketplace is as easy as uploading a zip file and filling out a form. Unlike the Google Play Store, all apps are QA checked before going online, so expect a 12-24 hour delay.
If you have a FirefoxOS device, you can install Protect Your Thing! for FirefoxOS.