Designed to integrate with the character of the FirefoxOS, the Fira typefaces also aim to cover the legibility needs for a large range of handsets varying in screen quality and rendering.
The Fira font family comes in a Sans Serif with 4 weights (Light, Regular, Medium and Bold) all accompanied by italic styles. The package also includes this Mono Spaced variant with 3 weights (Regular, Medium, Bold.)
See the Mozilla FirefoxOS Style Guide for more details.