Mail Merge doesn't work on versions of TB higher than 60.

Get TB 60 here.

This thread pretty much sums up beta versions and add ons. https://support.mozilla.org/en-US/questions/1246282

My Thunderbird to automatically update. This clearly was a bad idea.

You do not automatically get on the beta channel. Get the latest release version at Thunderbird.net

I've reinstalled TB 60. It just works! Later releases do remove important functionality. I urge the 'volunteer army' to address the loss of Merge mail functionality!

DavidTennant said

I've reinstalled TB 60. It just works! Later releases do remove important functionality. I urge the 'volunteer army' to address the loss of Merge mail functionality!

I don't know if it is still on the web site, but there used to be a disclaimer for daily build that they were no guaranteed to even run. Now Beta versions are usually much better than that, but they are not release quality. Given that massive changes Mozilla has made to the code base Thunderbird relies upon, the developers are n the process of basically starting the add-on interfaces from scratch. This is going to be slow and tedious and might even delay the release of Thundered 68 in July.

There is a hybrid API that is being used for beta, but it will not function in V68 so add-on developers are not putting much effort into making versions that will only ever work in Beta. However with donation having allowed the hiring of some new people. Perhaps some of the technical debt hat has grown up in Thunderbird can be cleared out and version 68 will be the leanest fastest and most secure version ever.