Thanks I'll file a bug if I see it again on a GitHub page. In fact, the last days I haven't noticed it anymore on GitHub pages...
But, of course as stated in https://bugzilla.mozilla.org/show_bug.cgi?id=1404468#c43 there a sites like http://www.jeuxvideo.com/ which huge (1-2sec) FOUC in Firefox (not noticable in Chrome).