Clear Cookies & Cache

• https://support.mozilla.com/en-US/kb/Template:clearCookiesCache

Clear the Network Cache

• https://support.mozilla.com/en-US/kb/How%20to%20clear%20the%20cache#w_clear-the-cache

Troubleshooting extensions and themes

• https://support.mozilla.com/en-US/kb/Troubleshooting%20extensions%20and%20themes

Check and tell if its working

Make sure that you allow pages to choose their colors and that you haven't enabled High Contrast in the Windows Accessibility settings.

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

No, that's not that. Something else has been changed. E.g., the browser seems to be working more slowly: Gmail can only load in Basic version, but times out in normal HTML version.

Did you check this setting?

• Tools > Options > Content : Fonts & Colors > Colors : [X] "Allow pages to choose their own colors, instead of my selections above"

If you uncheck this then you do not see background images like you posted.

Can you attach a screenshot?

• http://en.wikipedia.org/wiki/Screenshot

Use a compressed image type like PNG or JPG to save the screenshot and make sure that you do not exceed the maximum file size (1 MB).