See https://support.mozilla.org/kb/unable-launch-older-version-profile

Why are you running a beta anyway? These are intended for testing purposes, not for production.

Thank you for your response. I was on the beta channel and interested in new features but it has messed me up. Learnt my lesson but is there no way to roll this back now?

With Thunderbird 68, in the command line in the Thunderbird program directory, start with

 thunderbird.exe -P --allow-downgrade

Dear Wayne It worked - thank you - but I seem to have lost all my address books. Any idea how I can recover them?

A last question - I would love to learn more about the programming and different possibilities. Could you point me to a website or resource that I can learn more about some of this instead of just dabbling (dangerously)?

Thanks again Shamit

Do Help > Troubleshooting > open profile folder. Shut down Thunderbird. find the xxxx.mab.bak files and remove the .bak extension.

For programming, see https://developer.thunderbird.net/the-basics/building-thunderbird

Wayne Mery said

With Thunderbird 68, in the command line in the Thunderbird program directory, start with thunderbird.exe -P --allow-downgrade

I don't know what the above message means. Are you saying I have to open a DOS-type command line instead of using Properties?

Can you give me step-by-step instructions to roll back version 68.3.1?

Yes that is what you need to do. Start a command prompt (type com in Windows search) and navigate to the Thunderbird program directory (use chdir or cd command) Then run the command that Wayne has indicated

Thank you for your help.

Do I do that before or after I install the previous version? And what previous version should I go back to which will not break ALL of my extensions?

Also, once I run that command on the command line, what am I going to see, and how do I know which profile to select? Will I see a list of profiles?

Install the previous version over the current version but don't launch Thunderbird when you finish the install. Launch Thunderbird from the command line - the -P argument launches the profile manager, and the --allow-downgrade allows you to roll back to the previous version of Thunderbird.

Now that's the information I need! I don't suppose you know which version I should install to use my old extensions, and where I can get it (somewhere on this site, I assume).

What you will seen in profile manager is something like https://support.mozilla.org/en-US/kb/using-multiple-profiles

You can always get the current version from the main website https://www.thunderbird.net/en-US/ (although currently slightly behind - don't worry about it)