TechLifeBlogged

I wasn't going to cross post this but I think it is an important enough security/privacy problem that I think it is worth it.
From Firefox Help, Tips and Tricks:
There is a bit of a storied history with Firfox and 3rd party cookies. There used to be an option to disable them in the options dialog box but that was removed in Firefox 2. Fortunately you can easily modify the preference by using about:config.
In your address are, type about:config
Find the line that says network.cookie.cookieBehavior
Right-click on the line click Modify
Change the entry to a 1
Click OK
Restart Firefox

More details:
About the network.cookie.cookieBehavior from Mozillazine Knowledge Base
Recent thread about 3rd party cookies on Mozillazine Forums
Tags: mozilla, firefox, browser, cookies, how to, help