It would be nice if in the privacy and security settings instead of just being able to change the user name and password, you could also update the provider.

What do you mean? You can change anything in the settings including the account name and server details.

Under Preferences/Passwords/Saved Logins is what I am referring to. You can't edit the Provider information, only the user name and password. Though you can use text edit to edit the logins.json file, but that is more than the average user will know how to do.

After I posted the above response, I turned off the master password and restarted Tbird, issue persisted. Next I quit Tbird, and edited the logins.json file, changing the provider SMTP address. Restarted Tbird and sent a test e-mail, the dialogue box prompted for my password, but this time it allowed me to enter a password.

I managed to resolve my issue, but don't know why this occurred, as before the installation of Apple OS 11 beta, it functioned as expected.

A simpler solution would have been to delete the old provider's entries from the Saved Passwords dialog, forcing Tbird to prompt for a new password when you next check for or send a new message.

Agreed Stans, but that doesn't explain why a dialogue box prompting for a password, does not allow an entry to be made. It was only a workaround to the problem.

A dialog to modify an orphaned entry is non-responding, is enough explanation for me.