How many folders are there in the Roaming/Thunderbird/Profiles backup? If you have more than one, then you need to identify the correct one, then open it and inside you'll see a file called compatibility. It's a configuration file, so you can open it using Notepad. Paste its contents in your next reply.

Ok I have a zip file of the AppData/Roaming/Thunderbird folder from the computer when it was working.

The directory structure looks like this:

Thunderbird

 - Crash Reports
 - Profiles
      - aeinq54q.default
           - calendar-data
           - crashes
           - extensions

In the aeinq54q.default folder there are a bunch of files but they mostly seem to have different names than the files in the profile that was created when I installed thunderbird on the newly reformatted computer. The profile after re-installing is called "8cherwcu.default". I assume because I seem to have used a different user-name when I reinstalled windows.

To the specific question you asked: in my backed-up profile folder there is no file named "compatibility". Here is a list of the filenames contained within my backed up profile:

abook.mab

blist.sqlite

blocklist.xml

calendar-data

cert8.db

cookies.sqlite

crashes

directoryTree.json

extensions

extensions.json

folderTree.json

global-messages-db.sqlite

history.mab

key3.db

mailViews.dat

parent.lock

permissions.sqlite

places.sqlite

pluginreg.dat

prefs.js

revocations.txt

search.json

secmod.db

session.json

sessionCheckpoints.json

SiteSecurityServiceState.txt

times.json

virtualFolders.dat

webappsstore.sqlite

xulstore.json

Thank you for your help!

Greg

Your backup is incomplete then. You should have more folders like ImapMail and Mail in the backup folder, where your messages are actually stored.