Make sure you allow pages to choose their own colors and that the default colors are black text on light background.

• Settings -> General -> Language and Appearance -> Colors -> Manage Colors

Again, these are my settings:

 Windows: Light
 Firefox Website Appearance: Light
 Firefox Theme: Light or green (I now realize the themes only affect the top header)

Firefox Appearance -- Manage Colors: -- If I choose "Never" Override -- (i.e. do NOT override FF default colors with my chosen colors), I get: BLACK Background (Dark Mode) on everything. [Same result whether I check or uncheck the "Use System Colors" option). -- If I choose "Always Override -- which is what I understand you to be advocating), I get things whiter than white. For Example, on Outlook: I lose the colors of the red flags, the yellow highlighted flagged emails, etc. Nearly everything (except the top and left header bars) is ONLY white. And dropdown menus overlap (so text conflicts). Also there is no real local control within Outlook. Choosing for Settings: Use System Settings or Light mode makes no difference. Both show BLACK background, with white lines. Choosing DARK mode remains black, but removes the white lines.

[Meanwhile, back to Firefox Appearance Settings: If I select for background a non-white color, i.e. light lime green, nothing happens. If I deselect "Use System Color"), then everything previously all white -- or all black -- is now all light lime. 

How do I get NORMAL colors on Firefox, just like Chrome, Safari, and Edge? (I have No issues on other browsers -- just on Firefox). On the same computer, Firefox is giving me radically different results on the same webpages as my other browsers.

So, I go back to what makes "sense" and choose to NEVER override website colors with Firefox defaults (and everything set to LIGHT as above): All my websites are completely black again... Something is very wrong...