did you select to use "this copy" on all accounts when you installed in in the admin account? Otherwise a local copy in the user account will be installed and used.

Thanks for quick reply. No I wasn't aware of this option. I install from my User account but have to enter the Admin password to install. This is how I have done from the start of Thunderbird (about two years ago) and there have been many successful updates.

In the User account I have just reinstalled 91.10 using this route and custom, nowhere did i see an option to use "this copy". Again TB shows 91.10 after install then reverts to 91.9 on restart.

To be fair the version in the Admin account is showing 91.10 and in there I have just Updated with a new download of 91.10. Again nowhere did I see an option to use "this copy".

Am loathed to remove Thunderbird User copy as it has all my accounts and locally stored data. Any more guidance on overcoming this would be welcome?

I have solved my problem by removing both instances of the TB program [uninstall] from the Admin account [TB 91.10.0, and TB 91.9.1] As a precaution beforehand I backed up the non-admin User Profile just in case but it wasn't needed.

I then installed my previously downloaded TB 91.10.0 in the Admin account - again there was no option to select whether this should be all users or in fact any choices at all (other than alternate location for program)

I switched back to the User account and started TB and it picked up the profile and continued from where I had left off. All mail intact and as far as I can determine exactly as it should be.

The TB Troubleshoot Information reports its version as 91.10.0 [and still does after a restart] and there is now No update History (which is correct as this is now the first instance of the program)

Thanks for the hint about different copies of the program - though I have never seen options during installation in an admin account.

Windows differentiates between "an admin account" and "the hidden administrator account" https://www.techrepublic.com/article/how-to-enable-and-disable-the-administrator-account-in-windows-10/

I though I had solved it but....

Thunderbird has now updated itself to 102.0.2 (64 bit) without my intervention - despite the release notes saying it is only a direct download and not an upgrade. However, TB reminds me every day with a message that it cannot do an update and wants me to visit the TB site and download.

I assume some software switch is stuck and saying that there is an update when there isn't and posting the reminder. Is there some way of clearing this? I have cleaned the cache several times and clean installed the previous [91.10 version?] twice.