Yes, I've seen the same on GitHub. Note sure this is a GitHub or Firefox bug. (I use Nightly). Could you maybe file a bug at bugzilla for this?
In that bug you can see there are several things that can cause this, including website issues and certain extensions.
A bug report with steps to reproduce (even a link to the specific GitHub page involved would help) would be really useful if you are still seeing this. Please cc "bzbarsky". Also worth checking whether the problem persists with extensions disabled, if it's reproducible enough to do that with...
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).
Chrome has started just blocking the HTML parser completely on stylesheets. That means it doesn't end up with FOUC (except insofar as there is renderable content before a sheet that is affected by the sheet), but at the cost of less parallelism and slower pageload...