Clear the cache and the cookies from sites that cause problems.

• "Clear the Cache": Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"
• "Remove the Cookies" from sites causing problems: Tools > Options > Privacy > Cookies: "Show Cookies"

If images are missing then check that you aren't blocking images from some domains.

See:

• http://kb.mozillazine.org/Images_or_animations_do_not_load
• Check the permissions for the domain in the current tab in Tools > Page Info > Permissions
• Check the exceptions in Tools > Options > Content: Load Images > Exceptions
• Check the "Tools > Page Info > Media" tab for blocked images (scroll through all the images)

There are also extensions (Tools > Add-ons > Extensions) that can block images.

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

With Firefox 12 this cured by going to help, restart with add-ons disabled. Should start in safe mode with all pages displaying correctly. Then restart normally

Interestingly it fixed the same problem with other browsers too!