Is this with all websites, some, a few?

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

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

Can you attach a screenshot?

• https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem
• use a compressed image type like PNG or JPG to save the screenshot

Make sure you haven't enabled a High Contrast theme in the Windows/Mac Accessibility settings.

Make sure you allow pages to choose their own colors.

• Options/Preferences -> Content: 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.

This setting affects background images.

See also:

• http://kb.mozillazine.org/Website_colors_are_wrong