I am a novice with Linux, but I am aware that some distributions of Thunderbird install to different locations. Anyway, that's my guess. If Linux has a search feature for entire drive, search for the abook.sqlite file. That is the name of the addressbook, and the search may show you where the lost data exist.

Yeah, I can find two instances of abook.sqlite and two instances of abook.sqlite-wal. What I CANNOT find is any way to open these files to examine their contents or to get TB to point to one of these files instead of the current, empty, file. I assume there are corresponding files for the calendar and for loal folders containing archived emails messages?

My point was that, in finding a second copy of the file, you may have also discovered the lost profile. If the second copy is NOT in your current profile, it may be in the lost profile. Checking file size may be a tip that you've found it. Check the folder in which it resides and see if there is an imapmail folder and a mail folder.

One of the folders contains both an imapMail folder and a Mail folder; the other folder has just a Mail folder.

You're speaking of a "lost profile" and I think I understand what you mean by that, but I do not understand how to get TB to reopen that lost profile.

Sorry to be so dense, but I know less about Thunderbird than you know about Linux!

OMG!!! All I had to do was click the about:profiles link in Troubleshooting Information, and select the other profile - all my old stuff opened up perfectly! I have now set that profile as the default, and can move on. Thanks so much for your help!

GREAT!!! I'm glad it went so smoothly. THanks for the feedback. :)