Hi

Firefox now harmonises more elements with your toolbar theme. When the text color on your background tabs is light/white, it triggers Firefox to use a dark background on menus, the sidebar, the Library window, and various built-in pages. Firefox also signals sites that you have a dark theme preference.

You can download a different theme from https://addons.mozilla.org.

I hope that this helps.

The dark mode nonsense can be fixed by choosing the right theme or changing a preference. https://support.mozilla.org/en-US/questions/1364418#answer-1474290