It sounds like the dark mode issue in Gmail is frustrating! It seems to be related to a recent update in Firefox, possibly causing the setting you have for light mode to be overridden. Let's troubleshoot this step by step to help you get Gmail back to its light mode in Firefox. 1. Disable Gmail’s Dark Mode Settings Gmail itself has a built-in dark mode setting that could have been toggled. Here’s how to check and change it: Open Gmail in Firefox. In the upper-right corner, click the gear icon (⚙️) and select "See all settings." Go to the "Themes" section. Look for any mention of dark mode or choose a light theme if one is available. 2. Check Firefox’s Dark Mode Settings Firefox also has a built-in dark mode that can affect websites like Gmail. You mentioned that you have set layout.css.prefers-color-scheme.content-override correctly, but it’s not letting you change it. Here's an additional step to check: In the Firefox address bar, type about:config and press Enter. Search for ui.systemUsesDarkTheme. Right-click it and select Toggle to set it to 0 (which means light mode). Restart Firefox to apply changes. 3. Override Gmail's Dark Mode in Firefox’s about:config If the above steps didn’t work, you can force Gmail to display in light mode through a custom configuration in Firefox: In the Firefox address bar, type about:config and press Enter. Search for browser.display.use_system_colors. Set it to true (if it’s set to false). This forces Firefox to follow the system's theme settings. 4. Test Firefox Themes Sometimes Firefox’s custom themes can also affect the appearance of websites. If you have a dark theme installed for Firefox itself, try switching to a default light theme: Click the hamburger menu (three horizontal lines) in the upper-right corner. Go to Add-ons. Under the Themes section, make sure a light theme is enabled, or set it to the default theme. 5. Disable Firefox Add-ons If you use any extensions or add-ons in Firefox (like dark mode extensions or theme-related add-ons), they may be causing the issue. Try disabling them temporarily to see if Gmail’s appearance changes: Click the hamburger menu (three lines) in the upper-right corner. Go to Add-ons. Disable any add-ons related to dark mode or themes and see if Gmail behaves as expected. 6. Clear Cache and Cookies If none of the above methods worked, clearing your cache and cookies might help resolve this: Click the three horizontal lines in the upper-right corner of Firefox. Go to Settings > Privacy & Security. Scroll down to Cookies and Site Data, and click Clear Data.