OKAY! I may have solved my own problem, whew. I went and backed up that session.json.backup file again and reopened it and took another crack at those INBOX numbers. It appears, based on a quick random sample, that these numbers are the correct email tabs that I had open. '''The numbers refer to what ThunderBird calls "Order Received". ''' So, I'm going to provide a hypothetical solution for anyone else who a) had TBird close all your tabs and b) has by great good luck NOT closed TBird since that moment and thus has a session.json.backup file that still lists your open tabs from before the update or whatever Anyone who actually knows what you're doing, please jump in here with better information for later travelers on the same road... (These instructions are for Windows 10... I hope users of other systems can work it out from this.) 1. Don't close Thunderbird! 2. Hunt for a file named session.json.backup -- In my case, MS Windows 10 "File Explorer" Search would NOT find it. I've had search running for 30 minutes so far without luck. 3. I did find it by following the path listed below: C:/Users/YOURUSERNAMEGOESHERE/AppData/Roaming/Thunderbird/Profiles/RANDOMCHARACTERSLIKE0fms2q1234.default-release/session.json.backup 4. Right click that file, choose "OPEN WITH" Firefox. The Firefox browser will apparently open JSON files just fine. (Before you do that, make a copy of the file with a different name. Also, make a copy of the session.json with a different name. Finally, delete the ".backup" from session.json.backup. You will want the backup files, just in case, etc etc.) 5. With great good luck, you may see a long file of computer gobbledygook. It may have entries like this: >4: {…} in which case you click that little triangle > symbol right at the front and hopefully it expands to look like this instead: >4: mode: "message" messageURI "imap-message://YOUREMAILNAME%40gmail.com@imap.gmail.com/INBOX#3606" ext {} Or, it may open up with the longer version and you don't have to click the triangle. Either way, you want that one precious part: INBOX#3606 or INBOX#999888 or whatever. Each of those numbers corresponds to an email in your inbox. (You may have several inboxes, so pay attention to which one is being listed: gmail or whatever). (Aside, I believe that if it opens with a *short* list of gobbledygook then it does NOT have a useful backup for you and this method probably won't work: it means your backup file was made after the disaster, not before. I'm sorry, that really is unfair to you.) Now, you remember that number, and go open TBird. Up at the top of your inbox, you'll have a bunch of headings like "Subject", "Date", etc. RIGHT CLICK that heading bar and a pop up menu should offer you the option to add or delete some of those headings. A long list, and way down near the bottom, "ORDER RECEIVED". Check mark "Order received" and a new column should appear in your inbox... drum roll please, with all those nasty numbers in it! Scroll down and look for the message number 3606 or 999888 or whatever, and open that message in a new tab. I wish this was the wisdom of the ages, but actually, this is all I know. It seems sort of simple in hindsight, but it will only work if you've had a lot of luck. So, Good Luck! (And in future, don't keep tabs open like I did. "Tag" the message "To Do" so you can find it again easily. That's what I'll do from now on.) Warm regards, Craig (Still not a programmer since the 1980s...)