Strange Firefox problem (can't log into Yahoo mail)

I have this strange problem concerning Firefox 3.6 and logging into Yahoo mail (webmail) where Yahoo keeps asking me for the password but still won't let me in (yes, the password is correct).
Using Safari however works fine with the same password/username.

I even tried reinstalling Firefox, assuming the preference files or something was damaged.

 

No, it just occured to me that I hadn't tried it with a completely fresh Firefox setup, so deleted the profile folder and had Firefox create a brand new one, then copied back my backed up files one by one.
Turns out "permissions.sqlite" was the culprit.