It's not you, it's Firefox 96. Firefox 96 now harmonizes more parts of Firefox with your '''toolbar theme'''. When the text color on your background tabs is dark/black, it triggers Firefox to use a light background on menus, the sidebar, the Library window, and various built-in pages. Firefox also signals sites that you have a light theme preference. ''What to do about this now'' '''''(1) For people willing to change their toolbar theme''''' The fastest workaround for now is to change your theme to one that uses light/white text. By light text I mean, for example, the built-in Dark theme, or an add-on theme with light or white text on the tabs. '''''(2) For people who can't part with their toolbar theme''''' This is only a partial workaround for the built-in pages, and for websites that have light/dark responsive themes. Currently, the only way to decouple your page (content) theme preference from your toolbar theme is through the back door: (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button accepting the risk. ''More info on about:config:'' [[Configuration Editor for Firefox]]. ''The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.'' (2) In the search box in the page, type or paste '''layout.css.prefers-color-scheme.content-override''' and pause while the list is filtered (3) Double-click the preference to display an editing field, and change the value to whatever you prefer from the list below, then press Enter or click the blue check mark button to save the change. * 0 => Force a Dark background * 1 => Force a Light background * 2 => Follow the System theme (default in Firefox 94) * 3 => Follow the Browser toolbar theme (default in Firefox 95+) The about:config page should immediately reflect your saved change. Probably that carries over to error pages? ''What about other aspects of the UI?'' For the sidebar, menus, and toolbar drop-down panels, there is an unofficial, community-supported option, which is to override the built-in styles using your own style rules in a userChrome.css file. I think people are still pondering how to do that efficiently, and perhaps you aren't that into hacking your browser anyway.