After upgrading, 1 of my 4 accounts in a profile, one email account stopped being able to send. Noticed that the outgoing server info had disappeared. Re-entered it. Could send one email. Then Tbird failed to send that account.
I closed Tbird, reopened. Then several of the missing outgoing server entries for that 1 email account showed up (duplicates.) Was able to send 2 emails, but neither showed up in the "Sent" folder.
Deleted the email account, re-added the account. Working now.