Tbird looks to c:\users\(userID)\appdata\roaming\thunderbird\profiles.ini to tell it the name of the profile to open. Since you have the technical skill to rename, you may find it simpler to edit that text file (notepad can do it) to point to whichever profile you prefer. You will see multiple path statements. Just change all to point to your preferred one.

This took me back to the original problem and message ................ "Set Up Your Existing Email Address"

I can see that the Profile files hold all the emails, but can't work out how to get them back into TB when it opens?

Are you an Avast/AVG user? There has been an issue where it quarantines the prefs.js file in the profile folder, producing the effect you see.

https://support.mozilla.org/en-US/questions/1371932#answer-1491711

https://www.reddit.com/r/avast/comments/tkabes/avast_quarantined_thunderbird_emails_prefsjs_file/

I have the same problem. TB was working fine yesterday, all day. I did set up a new profile that I designated as the default profile.

Today when I open TB, it came up on the New Account Setup page. Now TB will only open to that page.

I am able to access my existing profiles by selecting the Home tab and using Help --> Additional Troubleshooting Information -- Profiles about:profiles and then launching my original profile (not the new one -- I'm scared to use it now). TB works normally afterwards but I have to do it each time I restart TB.

I checked the profiles.ini file as suggested by david and all of my existing profiles are correctly listed in the path statements.

Update to my previous post.

In TB, I changed my default profile back to the .default-release profile. For the past 2 days I have had no problem opening TB to this profile and having all emails, contacts, etc available.

When I first installed TB, I used the .default-release because I didn't know better and I was the only user of this PC. Later, after learning about profiles, I tried to create a new one to use instead of the .default-release profile. Worked fine during that session, but the next day when I opened TB is when my "lost profile" problem started. That profile is still on my system, but I am leery of using it.

So, this is no solution, just a work-around.