If you edited the source code, you will have to remove that version from your system completely and reinstall.

How do I do that?

You can create a new profile as a quick test to see if your current profile is causing the problem.

See "Creating a profile":

• https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles

If the new profile works, you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.

• https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

These may help.

https://support.mozilla.org/en-US/kb/uninstall-firefox-from-your-computer https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac

I tried to refresh Firefox but it's still coming back with some websites in dark mode and some not.

If your system theme is dark then this will be picked up by websites, you can avoid this by changing the website appearance setting to "Light".