I WANT TO USE THUNDERBIRD ON MY NEW COMPUTER BEFORE I STOP USING MY OLD ONE

#   start thunderbird and all should be there

This is not a problem. if the accounts are POP, you would need to coordinate somewhat, but that would be only concern. If account is IMAP, thee is nothing to be concerned about. If using Windows, these steps will copy to new pc: # ensure thunderbird is NOT running on old machine # enter %appdata% in search window on task bar and press Enter key # this should open windows file explorer in Appdata\roaming folder # highlight the Thunderbird folder and copy to external media # ensure thunderbird is installed on second computer, started once and NOT running # enter %appdata% in search window on task bar and press Enter key # you should be in Appdata\roaming folder , copy&paste the exported Thunderbird folder there. This WILL prompt to overwrite an existing folder of same name. allow it # start thunderbird and all should be there