and checkmark the box allow websites to set cookies
click ok then close Firefox and restart it.
Hope this helps.
I have been using Firefox for a long time and all of a sudden it will not set cookies or retain passwords. I cannot log into yahoo mail at all or I get a '999 Error' message. Under 'Options' it is set to accept cookies. I have no problems with IE - it's functioning normally. I don't recall changing any settings and I don't have any of my security apps configured differently than normal. I tried uninstalling and reinstalling Firefox, but no change. Any ideas?