Aha! I have had such trouble doing this, but realized (thanks to Elizabeth) that if you go to Firefox and click on Tools, then Options, and fix it so you check "Accept cookies from 3rd party sites" OR if you don't want it accepting all cookies, go to Exceptions and add www.blogger.com. This worked for me, and I hope it does for you, too.
Let's see if this works.
Aha - it was the third party cookies issue. I'll monitor it and try the exception if it seems to get out of control! Thanks!