Firefox OS is a pretty neat mobile operating system that is fully running using web technologies as its platform. Except the underlying kernel (called "Gonk") everything is running in the "Gecko" rendering engine (well known from Firefox).
If you got a Geeksphone, you may want to contribute to Gaia or you just want to adjust it to fit your preferences and needs.
Adapting it is fairly simple - you start with the setup process like described in this handy guide to flashing a new Gaia version onto your Geeksphone. If you have KEON and not a PEAK, leave out the
export HIGH_DPI=1 bit and you should be fine.
If you have some version of Gaia already installed on your phone, you may wanna skip the
make reset-gaia step!
Make sure that you checked out the branch that corresponds to the version of Gaia you have on your phone (e.g. v1.2, see Settings > Device Information > Software).
You can now happily hack on whatever you want to change, then run:
and after a short moment, the changes will be on your phone.