Well- they're giving back to the community besides being the only free, open-source, and non-invasive internet browser. When I booted up Firefox 4 today, the following greeted me from my start page:

Well- they're sort of "giving" back, in the sense that they gave us a really fun way to promote Firefox mobile with all of our friends. But hey, it's not like they're making money off of it, they've just gamified their own mobile browser. The game is, of course, free to download, and called Spark (you know, how you get fire). Spark is an inventive and creative incentive to share Firefox Mobil with your friends. The more you share, the more stats you get, the more badges you unlock, and the more fun you have! You also get to participate in this nifty thing- the Spark Global Visualizer. Make sure to watch the time lapse, its very aesthetically appealing, especially with that black background. This is nothing new for Mozilla, as they mounted a similar participation campaign with user's tweets creating a pixellated Firefox Logo.