Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or if userChrome.css/userContent.css is causing the problem.

• switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

I have this same issue. I tried the previous reply but it did not help. I also uninstalled and reinstalled Firefox to see if that would resolve it. It did not.

I have solved the issue. It is something to do with Firefox and sRGB compatible images.

Anyway, my problem was that my Alienware monitor supports HDR and Dolby Vision. This was causing the colours to go wrong with sRGB images. So, I switched the HDR off and chose the sRGB compliant setting in the Windows display settings and it then worked fine.

I now need to remember to switch HDR back on manually if I ever play something that supports it.