Firefox automatically enables High Contrast Mode when "Increase Contrast" is checked on MacOS. It can be turned off in FF settings under General -> the Colors... button -> Override the colors specified by the page with your selections above -> Never.

THANK YOU!!! That worked!!!

This issue should have been fixed with updating to 91.0.2 as in that version the default has been changed to Never, so updating should fix this unless you had already changed this setting from the default (about:config => browser.display.document_color_use is user set).

See the release notes.

• https://www.mozilla.org/en-US/firefox/91.0.2/releasenotes/