Last weekend I was in Firefox OS AppDay. I decided to go, because I knew nothing about Firefox Os and I thought it would be a nice moment to get started and for sure it was.
I found interesting several articles in hacks.mozilla.org
Also there is a tutorial (in Spanish) of how to build the snake game for Firefox Os, that it will be the next try to start with Firefox Os :-))
A video of the introduction given by Chris Heilmann (@codepo8) on the Firefox Os App Day
and a great tutorial of how to build apps with mortar