Newly released. To be standard in FirefoxOS. Ubuntu already has it in their repo, apparently. http://mozilla.github.io/Fira/
For what it is worth I have mozilla-fira-otf and mozilla-fira-ttf in my overlay (accessible through layman[mercurial] in the k_f overlay)
+*fira-sans-4.002 (07 Mar 2015) + + 07 Mar 2015; Ben de Groot <yngwin@gentoo.org> +fira-sans-4.002.ebuild, + +metadata.xml: + Initial commit. Fixes bug #485922.