I think not, but is it this: The [https://www.mozilla.org/firefox/91.0/releasenotes/ release notes for Firefox 91] mention this Mac-specific change: "Firefox now automatically enables High Contrast Mode when "Increase Contrast" is checked on MacOS." Is it possible you have that setting? ''If so, could you check what this does on Mac:'' When users of Firefox on Windows want to bypass "High Contrast" behaviors triggered by the system, they change a selector on the Settings page (formerly known as Preferences or Options), or you can modify a value in about:config. '''''Settings''''' In the "Font and Colors" section, click the "Colors..." button and change the selector below "Override the colors specified by the page with your selections above" to '''Never''' and OK that. ''See:'' [[Disable high contrast mode in Firefox]]. '''''about:config''''' (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button accepting the risk. (2) In the search box in the page, type or paste '''browser.display.document_color_use''' and pause while the list is filtered (3) Double-click the preference to display an editing field, and change the value to '''1''' then press Enter/Return or click the blue check mark button to save the change. Here's what the values indicate: * 0 => detect preference from system settings (default) to choose between page's colors and user's colors * 1 => always use page's colors * 2 => always use user's colors Any difference?