Same here. The last few days every website when viewed in Firefox shows what looks to be a stripped down version of the site - all colours around navigational elements are gone. See screenshot. Not happening in any other browser.

Interestingly, I completely uninstalled Firefox. Emptied trash. Reinstalled and, even before I signed in to my Firefox account (so, it was a completely vanilla install of Firefox), the same problem was there. (had previously tried safe mode and the helps on this page here: https://support.mozilla.org/bm/questions/1078342 all without any effect).

On Mac, big sur latest version and latest version of Firefox.

Follow this click sequence: Firefox\Preferences\General\Language and Appearance\Colours

• Now look for Override the colours specified by the page with your selections above
• From the drop-down menu, select Never.
• Click OK

As a precaution, I then exited Firefox and reopened the program. Webpage backgrounds now appear to be normal.

I hope that this works for you.

Follow this click sequence: Firefox\Preferences\General\Language and Appearance\Colours

• Now look for Override the colours specified by the page with your selections above
• From the drop-down menu, select Never.
• Click OK

As a precaution, I then exited Firefox and reopened the program. Webpage backgrounds now appear to be normal. I hope that this works for you.

Thank you! That was exactly what I needed to do

See also "New" in the release notes.

• https://www.mozilla.org/en-US/firefox/91.0/releasenotes/

Firefox now automatically enables High Contrast Mode when "Increase Contrast" is checked on MacOS in the Accessibility settings.