When you get strange, glitchy display artifacts, a strong possibility is that Firefox is incompatible with your graphics card drivers. Could you try disabling graphics hardware acceleration?

You need to restart Firefox in order for this to take effect, so save all work first (e.g., mail you are composing, online documents you're editing, etc.).

orange Firefox button (or Tools menu) > Options > Advanced

On the "General" mini-tab, uncheck the box for "Use hardware acceleration when available"

After restarting Firefox, is the issue resolved (or at least improved)?

If you can't access the menu bar in normal mode, you could try starting Firefox in its Safe Mode. Exit all Firefox windows (you can right-click the Firefox icon on the Windows 7 Task Bar to help with this) then hold the Shift key when starting Firefox.

In the dialog, click "Start in Safe Mode" (not Reset)

To proceed to make the change described above.

You can also try to clear the cache and the cookies from websites that cause problems.

"Clear the Cache":

• Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Firefox/Tools > Options > Privacy > Cookies: "Show Cookies"

Make sure that you allow pages to choose their own colors and that the default colors are black text on a white background.

• Tools > Options > Content : Fonts & Colors > Colors
  "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