I want to return to the normal contrast setting of white background, black text.

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh).
Is the problem still there?

Are you possibly using a dark theme or a reversed High Contrast theme in your operating system ?

You can check the default colors in Options/Preferences.

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.