FirefoxOS is a simple concept - every app is an Open Web App.
As of now FirefoxOS is stable and running on quite a few cool devices like this huge panasonic TV or this cool transparent high end Japanese phone.
Find a device for you or look at all devices
Building apps
Creating apps for FirefoxOS is as simple as creating a website in html. Because that is exactly what FirefoxOS apps are - webpages with access to phone features.