This could be a problem with High Contrast Mode on Mac. See "New" in the release notes. *https://www.mozilla.org/en-US/firefox/91.0.1/releasenotes/ Firefox now automatically enables High Contrast Mode when "Increase Contrast" is checked on MacOS in the Accessibility settings. <hr> Make sure you allow pages to choose their own colors. *Settings -> General: Fonts & Colors -> Colors: "Override the colors specified by the page with my selections above" => Never *https://support.mozilla.org/en-US/kb/disable-high-contrast-mode-firefox