Hopefully the "Thunderbird files" are your profile and I wish you'd asked before you started, because it's better to start by installing your old profile than to try to add it in later. So, is it your profile that you have on the USB drive? https://support.mozilla.org/en-US/kb/profiles-tb Right now, one way of making use of the old profile will be to import specific mailbox files and associated subfolders. I'd suggest the ImportExportTools add-on for this. https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/ You ''could'' copy in the old profile and set Thunderbird to use it, but you might lose whatever work you have done since you installed Thunderbird. If all your accounts are IMAP-connected, this may not actually matter, since your messages may well all be on the mail server(s), independent of any local storage. Swiitching to your old profile will restore all your old account settings, old messages, address books, filters and most of your add-ons, themes, dictionaries etc etc. All of the above is written with standard Thunderbird in mind. I haven't used Portable Thunderbird, but it must still use a profile and so the details about locating the working profile will be slightly different - it will, naturally, be on the USB drive itself. But wherever it is, the button found in '''Help|Troubleshooting Information''' will take you there.