I opened Firefox and it was bright white and I can't change it back. I never changed this so I am wondering why my settings were changed, I cannot use Firefox without using a dark theme.
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. It also signals sites that you have a light theme preference.
What to do about this
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 white or light text on the tabs such as:
I have attached a screenshot illustrating the difference between a theme with light text on the tabs and dark text on the tabs.
Does that work for you?
I explain about a solution for forcing Firefox to use a Light theme. Since you're looking to do the opposite and use a Dark theme, it'll be the same process but just use a value of 0 instead of 1 (as my post explains).
Hope this helps!