janharders - 2:59 pm on Oct 7, 2012 (gmt 0)
Install the Live HTTP Headers-Addon [addons.mozilla.org] for firefox, then Launch the viewer (Should be in the Tools-Menu after install & firefox restart) and surf your site where you expect to get cookies set. You will see the headers of the HTTP communication and can see what your script actually sent to firefox, and with that it'd also be much easier to help you diagnose what is going on.
BTW, just a shot in the dark: are you sending the cookies on a redirecting response? I've done that once and firefox did not accept my cookie until I've added an expiration date that is at least 2 hours from now.