First, if any of the accounts are IMAP, do NOT log on the site, as if the messages there are gone, Thunderbird will promptly delete the messages on the PC. Here are suggested steps:

1. there is text file named profiles.ini in the thunderbird folder. browse it and it will name the active profile's name
2. that folder will be in the thunderbird\profiles folder
3. imap accounts will be at imapmail\accountname and POP accounts at Mail\accountname and local folders at Mail\Local Folders. Message folders are generally in pairs of two, such as inbox and inbox.msf. The msf files are just indexes and Thunderbird will recreate them. (WARNING: depending on the status of your IMAP account, the message files may just contain headers. If so, the content is lost, but I am hopeful that the messages had fully downloaded.)
4. your addressbook will be abook.sqlite and your collected addresses will be history.sqlite, both in the root folder of the profile.
5. create a new Thunderbird profile and I recommend installing the importexporttools addon.
6. for neatness in organizing your files and folders, I suggest this:
   • highlight Local Folders, rightclick and create a new subfolder for each account
   • for each account, highlight the folder with account name, rightclick, select the importexporttools option to import mbox
   • then, locate a saved message folder for the account and press enter key. Repeat for eachmessage folder for the account.
   • when done, you will have each account listed under Local Folders and within each, the related message folders
7. for the addressbooks, you can click tools>import>import a file and proceed through menus to the sqlite option (another option is to just copy and paste them directly into the profile when thunderbird isn't running, overlaying the default files)

1. once all that is done, you may want to reconstruct your files to their accounts. That can be done, but I'm tired of typing. Let me know how it goes.

David,

Thanks for the detailed instructions! I will make the attempt later today or tomorrow when I have time away from work to focus and get it done right. I can confirm that when I checked the messages on the Thunderbird app running on the PC when offline, the content was there, so I assume all the messages are complete and to be found in the folder you referenced.

Will not log on to Thunderbird from the office PC until I am 100% certain that all the formerly deleted messages have been successfully downloaded into the current version. Will advise you as soon as I execute.

- Arthur :)