I love Firefox and it is my primary browser for years, on both - Mac and Windows, and it's not going to change anytime soon. Still, I feel Mac version is noticeably slower. Even though I disabled all extensions I can see the difference between Chrome and Firefox on Mac. Sometimes I can even see "naked" plain text page for a moment, before styling is applied (ex: on GitHub pages.)
This is called "Flash of unstyled content", and has been a bug since 57: https://bugzilla.mozilla.org/show_bug.cgi?id=1404468 . In 57 the delay before initial drawing was removed, which mostly makes things feel faster, but sometimes makes things show up naked for a moment.
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...
Yes, that one is on my to-investigate list already. Thank you for the reminder!
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...
