https://thunderbirdtweaks.blogspot.com/2019/09/i-lost-my-profilemail-on-update-to.html

Following that advice I get a dialog box saying

"A newer version of Thunderbird may have made changes to your profile which are no longer compatible with this older version. Use this profile only with that newer version, or create a new profile for this installation of Thunderbird. Creating a new profile requires setting up your accounts, calendars and add-ons again."

The last sentence of that dialog is exactly what I want to avoid having to do (what a waste of time it would be when things ran fine before I changed the choice of Linux distro)

Did you read the last past of the topic about downgrade protection? I think not as it addresses exactly that message and tells you how you how start Thunderbird to allow the downgrade.

I did see that but I had the same problem as the anonymous commenter of November 21. However, in view of your further response - for which many thanks - I followed Ariana's comment and then running thunderbird -allow-downgrade has apparently fixed things just fine. (The instruction to change to the Thunderbird installation directory and then run the command as ./thunderbird.... is not necessary for Linux users whose paths are set up so the command above will work.)

mahmacc said

(The instruction to change to the Thunderbird installation directory and then run the command as ./thunderbird.... is not necessary for Linux users whose paths are set up so the command above will work.)

Hmm, and who has their path set correctly. My experience is that the more people need help the less likely they are to even know what a path is. That is certainly the case on Windows where very few appear to even have heard of the path. It is still there and works like it always did. But the user base of windows has no idea.

So after adding -allow-downgrade to the launch string. What next?

I have been using Thunderbird for 10 years, my profile is a little over 5gb. When I added the allow downgrade switch it fixed the problem, but does that mean I will forever use the older profile structure? Will future versions of Thunderbird support it? Is there a way to fix the profile so Thunderbird 68.7+ can use my profile without needing the -allow downgrade switch?

Thanks

Tim said

So after adding -allow-downgrade to the launch string. What next?

Nothing, you have downgraded your profile. You only use it once.