Since there is no such thing as a Thunderbird account I am confused at what you are trying to do. Is the new account with the new provider a different email address or are you just changing email hosts for your existing address? If a new address with a new provider just add the new providers details to Thunderbird and start getting your mail as usual. '''Important.''' Before you remove the old providers account info from Thunderbird, you need to create folders under Thunderbird Local Folders and move any keeper emails into those folders. Any messages not in a local folder when you remove the old account info will disappear with the old account. The address book is part of Thunderbird. Exporting it for a backup is a great idea but was not really necessary just to change email providers.