First of all, if you're not browsing the web with third party cookies disabled please stop reading and disable third party cookies for your own protection. Third party cookies (TPC for the remainder of this blog) are generally a bad thing. Don't believe me, listen to Security Now!

Unfortunately, you may have noticed that some of your extensions broke after disabling TPC. There are a number of extensions (add-ons) which require TPC but it's always safer to whitelist trusted sites than to try to blacklist all the bad ones. Here are some extensions which require TPC exceptions and the domain required for the exception. You should try to whitelist the most specific level domain (subdomain) where possible.

Foxmarks
Exception URL: foxmarks.com

Seatbelt
Exception URL: pip.verisignlabs.com

Google Reader Notifier
Exception URL: google.com
Caution, this one allows all Google cookies, even adsense, but we trust them right?

Delicious Bookmarks
Exception URL: del.icio.us

For information on how to add a cookie exception please see Cookie Exceptions Window on the Mozilla support site.

If you use other extensions which require third party cookie exceptions let me know.

Update - 2008/08/28 - I just learned that mozilla requires a third party cookie exception for the site mozilla.org when installing certain extensions (addons). Some extensions require that you login to mozilla before installation. If third party cookies are disabled, the "install" link for these extensions will download an XPI file but the file contents will be an HTML login page. This is because the target page cannot read your login cookie so it sends you an HTML page instead of the extension. This results in the message "Firefox could not install the file ... because: Invalid file hash".