There is no simple way to ''merge'' profiles, unfortunately. In your position, I would import the mbox files into the new profile from the old one, using https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/ Browse your old profile and get familiar with its contents. You'll find a folder tree for each account, and each will have a series of files with names like "Inbox" which represent your old folders. You do not need the accompanying index files such as "Inbox.msf". I would do my best to avoid being in your position, by starting any new installation of Thunderbird with a copy of the old profile, rather than letting it create a new empty profile. Perhaps next time…