First, go to TB menu > General > Updates and change 'Allow Thunderbird to' to 'Check for updates, but let me choose whether to install them'. Then

- open your profile folder: TB menu > Help > Troubleshooting Information

- under the Application Basics section next to Profile Folder, click 'Open folder'

- close TB

- install the ESR version from https://www.thunderbird.net/en-US/thunderbird/all/

- close the install dialog - don't start TB from there

- delete the compatiblity.ini file in the already-opened profile folder

- start TB

Thank you very much, Ed. I've read that when switching from the Release version (more frequently updated) to the ESR version (behind release, version-wise), one must take certain steps to ensure that the (newer) release profile can be read by the (older) ESR version otherwise it will not be compatible with ESR. Perhaps you've covered that concern with your instructions to "delete compatibility.ini file"), but just want to make sure. Also, I've read that one can simply copy the Thunderbird folder under Roaming at %appdata% and simply copy that into a newly installed ESR version. Would that also work? Would it cover the profile compatibility issue?

> Perhaps you've covered that concern with your instructions to "delete compatibility.ini file"

Yes.

> copy %appdata%\Roaming\TB into a newly installed ESR version. Would that also work?

Yes, but there is no need to unless you want to use both release and esr.

> Would it cover the profile compatibility issue?

Yes, but only after you delete compatibility.ini.

Got it, but haven't yet tried it. Thanks.