You can check the default color.

Make sure you haven't enabled a High Contrast theme in the OS settings.

Make sure you allow pages to choose their own colors.

• Options/Preferences -> General: Fonts & Colors -> Colors: "Override the colors specified by the page with my selections above"

Try "Never" if the default "Only with High Contrast themes" isn't working.

Hi Mike, if the first reply doesn't resolve the issue, what pages are affected? Some Firefox internal pages like the built-in home page and new tab page will use a reversed color scheme (light on dark) if you have specified a dark system theme. There is a way to override that, but it's mostly for built-in pages, not web pages.

Or if the problem is with toolbars, see: Built-in themes in Firefox - alternative to complete themes.