Restarting browser in safe mode doesn't help.

Can you get to your Firefox profile folder using any of the methods mentioned in this article:

Profiles - Where Firefox stores your bookmarks, passwords and other user data

Note: If the article doesn't automatically detect your OS, in the left column, expand editing tools, and you can use the OS switcher there.

Your settings for text color, background color, and whether to observe system theme settings or always use the page setting are stored in the prefs.js file, which is a text file. You either could:

(A) Abandon all the customizations in the file by hiding it from Firefox (e.g., rename to prefsOLD.js) or

(B) Open it in a text editor and remove/modify the following lines:

To strictly honor page color rules instead of using system values, edit this line to a value of 1 or add the following at the bottom of the file:

user_pref("browser.display.document_color_use", 1);

To use default text and background colors, delete these lines -- note that your color values will vary from these examples:

user_pref("browser.display.background_color", "#ffff80"); user_pref("browser.display.foreground_color", "#400040");

The next time you start Firefox it should pick up those new customizations. Any improvement?

Did you try the Color settings page in Options/Preferences ?

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 -> 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.

Thanks, @jscher2000. Changing my preferences manually solved the problem.