I found this: http://postmaster.comcast.net/mail-error-codes.html 421 - [Too many sessions opened] Comcast allows 25 simultaneous connections per sending IP address. This error results when that limit is exceeded.

Therefore the situation is your logged in more than than the limit and not been logging out. Perhaps you are logging in using various methods.... am guessing your phone is also getting mail, or a laptop and the number of uses of your username and password has reached a limit.

Firefox is a completely different program and nothing to do with the issue other than have you used it to access webmail account and not been logging out ?

Yesterday night i spent six hours troubleshooting this issue, including one hour on the phone with Comcast. As i mentioned, Comcast is innocent because the same problem occurs if i DON'T use comcast's smtp server. I hacked around and i got convinced that Thunderbird was not even trying to talk to the smtp server: the Comcast error message was coming out before Thunderbird tried to send the email, as if it was set to respond that way regardless of what the smtp server said. I finally pinpointed Firefox as the cause when i noticed that the only change to my system files had occurred with the Firefox upgrade exactly one minute before Thunderbird started displaying that odd message. I have now removed Firefox and reinstalled it and guess what Thunderbird works again. I never touched any setting on Thunderbird other than trying different smtp servers. Not a coincidence to me that reinstalling Firefox solved the problem. It is my conclusion that either some malware was installed with the Firefox upgrade (whether because of Mozilla's poor security or because of my poor security) or that the Firefox upgrade affected some register values or godonlyknowswhat that led Thunderbird to believe that 1. i was using the Comcast smtp server even when i wasn't and that 2. i had too many sessions open on such server. if it's malware, i am sure that it will come back. So far it hasn't. If i am the only one ever reporting this issue, case closed. If someone else reports this issue, hopefully they will find this useful.

Hmm, My guess here, and it is a guess is that in that 25 count is web mail sessions initiated by Firefox. The update may have been not good with the result that Firefox never actually shut down properly to release those connections.

Uninstall will have forced a fix, just as rebooting the machine probably would have, by removing the bit that was still hanging in memory