The profile location is specified in the profile.ini file in the Thunderbird folder 2 levels above where you are looking.

If you left the locations alone in your old install, copying that over to the new system should work a treat.

Thank you Matt but I'm afraid I don't understand your reply! I did indeed leave everything alone when copying over to the new system - didn't dare do anything else! But as I say, though I've put the migrated file in the same place as the newly created profile file, the newly downloaded Thunderbird programme is accessing that new file, not the one I have migrated across. I am accessing these two profile files via C:\users\"myname"\AppData\Local\??\profiles\ In the Profiles folder are the two files with the randomly generated titles, containing all my account info - the new one and migrated one, both marked default. I suspect the Windows search isn't giving me the full pathway - hence my ??? above. If I try to move back up the pathway using the cursor, Windows just starts a new search and gives me the same info as above. Are you able to please a) give me a full pathway to find the Profile manager please and b) when I do find it will that enable me to make my migrated file the default one?

Would it work if I just deleted the new Profile file? Would that force Thunderbird to use the one I want it to?

I see you have local in the path. that is the problem... you are in the wrong place.

The best way to locate the correct path in Windows is to press the windows key and R then type %appdata% and press enter. This will place you in the correct "root" location, regardless of your windows version.

This will most likely be. C:\users\"myname"\AppData\Remote\

The folder the profiles ini file is located in if the above is correct will be C:\users\"myname"\AppData\Remote\Thunderbird.

Thank you Matt I tried your advice above but the path Windows insisted on was .....\Appdata\local\Thunderbird which indeed took me to the two profile files - It didn't recognise the path if I put Remote in as you advise above. There was though no sign of anything called Profile Manager. So I then tried a piece of advice I had seen elsewhere i.e. I copied and for insurance/back up saved elsewhere the two profile files ie the new one and the old one with all my info in it. I then deleted the contents of the new profile file and copied and pasted into it all the info from the old profile file., It was all there- I saw it all going across, subfolder by subfolder! I then deleted my original profile file leaving just the new one which now has all my info in it. But still, when I open up Thunderbird there is no sign of my subfolder system, old email, address book etc. It's there in what is now the only profile file but Thunderbird can't access it!

Maybe that should be "Roaming" not "Remote"?

Thank you Zenos. Interesting you should mention that as the friend who introduced me to Thunderbird suggested that too! Am having problems getting into the Windows 10 computer at the moment - this is not my week! But when I do, I will try looking for .......\Appdata\Roaming and see if moving my profile file there will do the trick.

Here on this Windows 10 PC it's

C:\Users\<me>\AppData\Roaming\Thunderbird

There is a button under Help|Troubleshooting Information that will take you to the data folder part of the current profile. Any other profiles should be found one level up, in a folder named Profiles. Alongside Profiles you should find your profiles.ini file too.

- Thunderbird
 profiles.ini
 - Profiles
  + abc123.default
  + def456.default

Or, to put it another way, the actual paths here are (with my name redacted):

"C:\Users\<me>\AppData\Roaming\Thunderbird\profiles.ini" "C:\Users\<me>\AppData\Roaming\Thunderbird\Profiles\<profile_name>" "C:\Users\<me>\AppData\Roaming\Thunderbird\Profiles\backup"

( I keep a backup copy in the backup folder. I also re-name my working profile folder so I don't get flustered by those random strings.)

I now have Thunderbird up and running with all my old stuff available to me. The correct path is indeed as in Zenos's answer above "C:\Users\<me>\AppData\Roaming\Thunderbird\Profiles\<profile_name>" I can't see any "profiles.ini" file but as everything is now working normally, I'm not bothered. Many thanks to Zenos and Matt for your advice and I hope this thread is helpful to other Thunderbird users.