It just updated, and I couldn't believe my eyes. The background colors on web pages: gone. Everything was high-contrast. The lack of background colors delineating sections of pages was disorienting. And many of the FF preference menus were actually unreadable. So what happened?

>Firefox now automatically enables High Contrast Mode when "Increase Contrast" is checked on MacOS

>Firefox automatically detects if you are using a High Contrast theme and displays everything in your high contrast color scheme. This overrides all other other browser or web page settings, and it affects the Firefox interface itself (all menus, windows, and dialog boxes) and the content of any web page you visit.

Yes, I have some "Increase contrast" enabled on Mac OS because the Mac OS dark-mode theme is too low contrast.

But I didn't ask for all my web pages to displayed like that. Yikes.

I eventually found that I could override changing colors in the advanced colors menu.