I will assume you use Windows If you installed from Thunderbird.net, the profile is normally at

c:\users\<yourid>\appdata\roaming\thunderbird 

A simple way to locate it is to follow these steps;

1. click help>troubleshootinginformation
2. scroll down left side to 'profile folder' and click 'open folder'
3. that will normally show the profile in the place described earlier.

If that is true, here are sample steps to backup and restore profile:

BACKUP

1. ensure thunderbird is not running
2. enter %appdata% in search window on task bar and press Enter key
3. this should open windows file explorer in Appdata\roaming folder
4. highlight the Thunderbird folder and copy to external media

To locate Thunderbird, if there is a desktop icon, rightclick and select 'properties.

RESTORE

1. ensure thunderbird is installed and NOT running
2. enter %appdata% in search window on task bar and press Enter key
3. delete the thunderbird folder
4. you should be in Appdata\roaming folder , copy&paste the exported Thunderbird folder there.
5. start thunderbird and all should be there

My apologies for taking so long to come back and report; much of the last month has been taken up by a sooner-than-expected short-distance move (and you do NOT want to know how many boxes of books are lined up in my new garage Very Large Closet™ waiting for decisions to be made about how many new or transplanted bookcases will go where).

At any rate: I discovered after further poking at File Explorer that I had done what was, with benefit of hindsight, a silly thing when I installed the ESR version of Thunderbird - I'd downloaded it from the Microsoft Store rather than direct from Mozilla/Thunderbird. As a result, the program files were buried in what struck me as rather scattershot fashion in a Microsoft chain of subdirectories.

This was, as one might imagine, a non-optimal state of affairs;, but I eventually did find a Thunderbird profile that looked likely to have all the right stuff in it.

With that in hand, I made a command decision, and downloaded the current Thunderbird into its own drivespace - which is to say, I left the ESR version where it was rather than deleting or overwriting it. That turned out to be helpful; while restoring the exported profile into the fresh installation got 95% of the settings properly right, there were just enough odds and ends that needed tweaking that having the old version available proved useful for getting the last bits of the new install fully configured. (I had expected just about that much slippage, as I have a fairly complicated cluster of mailboxes set up.)

Thank you very much for your post; while I ended up doing most of my own legwork, your instructions gave me enough clues to figure out where the data I needed was hiding, and that was a forward push I very much needed in order to carry out a successful transition from the ESR version to the mainline program.