I noticed this first when I had to force Google to go into dark mode even though firefox was already in dark mode. I have searched through some issue reports and have seen a lot of people mentioning that their system theme is not being detected properly. This is not that issue. I can set the firefox "Website appearance" to dark or automatic at which point the UI of firefox itself does do into dark mode so no issues there. However if I go to a website that tries to query 'prefers-color-scheme' and set its theme accordingly it always returns 'light'. This is the case even if I set the 'layout.css.prefers-color-scheme.content-override' to 0 in about:config. I used this website to test: https://septatrix.github.io/prefers-color-scheme-test/ The issue persists when opening a website in a private window. Let me know if I need to provide additional information. -Felix