For each account, there should be an smtp server with the same User Name. If you have accounts A, B, C and D, create servers in the Tools/Account Settings, Outgoing Server (SMTP) section, with Descriptions smtpA, smtpB, smtpC and SmtpD. They can all have the same settings if they are on the same server, but the User Names must be distinct, and correspond to accounts A - D. Next, select each account in the left pane of Account Settings and click the Outgoing Server (SMTP) drop-down in the right pane so account A sends on smtpA, account B sends on smtpB etc. Finally, remove all items from Saved Passwords in Preferences/Privacy & Security, restart TB, enter the password for each account when asked.

https://support.mozilla.org/en-US/questions/1324013

Thanks for your detailed reply @showes. I'm afraid I didn't have time to test it. An upgrade has corrected the issue in the meantime - I'm now on 91.9.1 and things are back to normal.