Yay! Solved the issue of the Help Center pages flashing, reloading in Firefox: requires disabling "Hardware Acceleration" in Performance Settings. Safari, still no idea.
https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems?redirectslug=Troubleshooting+extensions+and+themes&redirectlocale=en-US#w_the-problem-does-not-occur-in-troubleshoot-mode