Based on FF Meta, Fira Sans is the original font used by the Firefox OS and which Mozilla makes it available to the public.
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 a Mono Spaced variant with 2 weights (regular and bold) and is available to be downloaded in OpenType, TrueType or WebFont variants, including text CSS.
Download
http://www.mozilla.org/en-US/styleguide/products/firefox-os/typeface/
Usage example
