It looks like Firefox is using white text on a white background. Which theme is that? Maybe it needs to be updated to trigger Firefox to use a dark background in those panels.

Meanwhile, you could use one of the built-in themes. See: Use themes to change the look of Firefox.

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

Make sure you allow pages to choose their own colors.

• Settings -> General -> Language and Appearance -> Colors -> Manage Colors
  "Override the colors specified by the page with your selections above"

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

• https://support.mozilla.org/en-US/kb/disable-high-contrast-mode-firefox