Calling it a fork would be too much; it is actually just a modified main System application for FirefoxOS to allow it to run 'headless'. The rest of the infrastructure is nearly unmodified B2G.

Firefox OS is (basically): AOSP kernel, Boot to Gecko, Gaia frontend JanOS is: AOSP kernel, Boot to Gecko, JanOS frontend

Made some changes to both AOSP and B2G, but not so much. See http://janos.io/about.html