This used to be a simple question, now it has issues within issues., Like what was the source of the installation.

FlatPak on Linux and the Windows App store represent special issues. Likewise if the last revision of Thunderbird used with the profile was greater than the one installed another set of issues arise.

The real fundamental issue is what exactly is it you are trying to preserve? Is it something that the imminent release of Thunderbird sync (very like Firefox sync) will not cover?

It's hard to tell what thunderbird sync will or won't do. The latest update I can find on that are nearly a year old.

I assume an account sync will require an account and logon. I'm trying to provide seamless fault tolerance to my 73 year old mother. Usernames and passwords are not desirable. neither are complicated instructions. She just lost her brand new desktop and data to hardware failure. So while I'm setting everything up again, I want to get that sorted. I've already moved her profile on to an external drive, and configured a second local profile. I just need it to fail over to the local copy if the external copy is either not connected or lost. Seems like the best option is to create a batch job to check the primary if not switch to backup using 'thunderbird -p'

Seems like the best option is to create a batch job to check the primary if not switch to backup using 'thunderbird -p'

That may well be the best result, at least for now. I really do not know how you will account for the profile per install which appears to me to randomly loose/discard existing profiles on update and create a new one and ask for an email and password to start over

hmm thanks interesting points, I was not aware. So I've removed all profiles from profile manager. I've switched to using the -profile switch. which uses a profile at a specified location even if thunderbird profile manager isn't aware of the profile. I've also set the local backup profile to mirror the default profile at logon.