When TB connects to the account, you will be prompted to enter the (new) password. Check the box to have the password remembered, and then it will be stored in Saved Passwords. If the checkbox doesn't appear, reply for ways to restore it. For some services like gmail or Yahoo, the password isn't stored, but instead there is an 'oauth' token. If the account password is changed, remove the token, restart TB, and enter the new password in the OAuth browser window, and a new token will appear in Saved Passwords.

Error message doesn't come up with password option tick box to save how to restore it please?

This is for a client I'm trying to help. The retain password option appeared after she shut down & reopened Thunderbird. BUT she has accidentally added a 2nd email accounts in the same name whilst trying to edit the settings on the existing account & when she tries to click on one to do anything she gets the message that this account already exists. Then when she tries to remove one via account actions it doesn't give her the remove option.

She has shut down Thunderbird & opened again & an email came in to one of the accounts, but when she went to reply it said SMTP settings wrong, so she went in & edited those to be correct but now it won't let her do anything at all, can't even open account as she gets the original already have an account in this name message.

I'm trying to help her from a distance.

Any idea how to fix this mess?

The 'account already exists' error is caused by the Account Name being the same for multiple accounts. First thing to try is right-click an account in the Folder Pane, Settings, and change the Account Name. The default is the email address, which causes the error, so just change it to something unique, e.g. MyBusinessAccount.

If that doesn't work, there is a method that requires editing the preferences in Config. editor (Options/General/Indexing). Type mail.server.server1.name, or mail.server.server2.name, or mail.server.server3.name or..... into the search, and you will see the value as the Account Name. Right-click, Modify, to change each name to a unique name (except for the Local Folders account). That should make Account Settings accessible.

Thanks will get her to give that a go.

Client has sent this screen shot of what she gets when she selects remove account, she isn't sure which option to select & I don't want to advise her incorrectly - she wants to completely remove the account in question.

If the Remove message data is unchecked, the account is removed but the downloaded mail is retained in case it's needed in the future. Removing an account does not delete the mail that is currently stored on the mail server, it just removes the account from TB.