Here is the site description for Mozilla.com/Firefox
"Official site of the open-source browser. Includes product downloads, release notes, features overview, and information about switching from other browsers."
I don't know where someone got the impression that firefox wasn't open source but it very much is (gpl and everything). You can download the firefox source from http://releases.mozilla.org/pub/mozilla.org/firefox/releases/22.214.171.124/. If I recall correctly (and I may be way off) but I believe firefox (which was called firebird at the time?) was basically a browser rewrite of mozilla, with an emphasis on decreasing bloat. Then at some point the firefox source made its way back into mozilla.
There is simply no way the mozilla foundation could get away with rereleasing firefox outside of the gpl. There have just been way to many community contributors who would loudly object.