Can you confirm that if you manually navigate to google.com you see the Gmail icon as you would expect? If this does work, you might be seeing Firefox's built-in home page which looks a lot like Google's home page, but isn't actually the same page.

If the little test above worked you can try this:

• Go to google.com
• Open Firefox preferences
• Select the "General" tab
• Change the "When Firefox starts" option to "Show my home page"
• Click the "Use current page" button, and close preferences

Can you attach a screenshot?

• http://en.wikipedia.org/wiki/Screenshot
• https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

• Use a compressed image type like PNG or JPG to save the screenshot
• Make sure that you do not exceed the maximum size of 1 MB

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

Make sure that you allow pages to choose their own colors.

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

Note that these settings affect background images.

See also:

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