Screenshot of the difference?

Screenshots - you can Safari, Chrome and the original exported file are consistent. Firefox - oversaturates and skews towards red skin tones on playback in Youtube.

You can try to disable hardware acceleration in Firefox.

• Options/Preferences -> General: Performance
  remove checkmark: [ ] "Use recommended performance settings"
  remove checkmark: [ ] "Use hardware acceleration when available"
• https://support.mozilla.org/en-US/kb/performance-settings

Close and restart Firefox after modifying the setting to make the change effective.

• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

I tried the "hardware acceleration" setting to no avail.

Try to disable color management to test if it is caused by a problem with color management. You can set the gfx.color_management.mode pref to 0 on the about:config page to disable Color Management.

• gfx.color_management.mode = 0

You need to close and restart Firefox to make the change effective.

See also:

• http://kb.mozillazine.org/gfx.color_management.mode

Tried that already - unfortunately no change. Also tried a webgl property change I saw somewhere else on the fourm. Some say to download new AMD video drivers, but this is MacOS, so no option there.