You should create a secure mail key and use it in place of the normal account password. If you have TB remember passwords, make the change in Tools/Options/Security/Passwords/Saved passwords, right-click an entry, Edit.

I am well aware of the master password which I have already created and am using it now. However, it is not clear if this will overcome any changes UVERSE is planning after the 20th of this month. I can only assume my signon will be transparent if I am using the master password after that date.

Hi shydubren, this is not related to your Thunderbird Master Password. This is related to changing your account password in Thunderbird from your normal password to a key generated on their site.

In the support article linked by sfhowes, under "Create a secure mail key" Step 10 is:

"Go to your preferred email app and replace the existing password with your secure mail key. (For an IMAP account, delete the existing password for both the IMAP and SMTP servers and replace them with your secure mail key.)"

Strangely, my original password started to work again. The Master Password did not work and I could not find it in my list of all passwords. I will continue with my old password until that stops working and then create the Master Password once again.

Keep in mind that a master password won't appear in the account password list since its purpose is to protect access to that list. If you forget your master password, there is a way to reset it - but that procedure will erase all account passwords.

http://support.mozillamessaging.com/en-US/kb/master-password