Hello Firefox OS Enthusiasts. Mozilla has relesed an early version (alpha version) of Monster Madness Online, a multi-player zombie-shooting game, playable via the internet browser.

Monster Madness is a PC and mobile game developed NomNom games, a subsidiary of Trendy Entertainment, that has been recently ported in HTML5, to work as an in-browser game, by Mozilla and the Trendy Entertainment developers.
The first stable version of the game will be released in May 2014, on the Monster Madness’s official site, and will be playable from Firefox, Chrome and Opera. Everybody can play the game for free, but some users may prefer to pay, in order to upgrade their characters.
For more information, see the above video:
Off-topic but related:
2013 was a good year for Mozilla. While they have constantly updated Firefox and Thunderbird, they have also created Firefox OS, a mobile operating system based on Meego Linux, using HTML5 applications.
An interesting infographic has been uploaded on the Mozilla blog, marking the most important achivements of 2013.