Maybe you can try to update that add-on? If automatic update doesn't show anything search it and install it manually

Thanks, I tried that. I have checked for updates. This is MailMerge 7.2.1 which says it is fully compatible with Thunderbird 78. Obviously not, unless something very odd is with my profiles.

Yep, profile files might have been corrupted. You might try these steps: Click the Application menu button, then go to the Help menu and choose Troubleshooting Information. A new tab will open with information about your setup. Click on Open Folder -close TB -copy (do not move nor delete) prefs.js file(s) (might be multiple pref-nnnn.js files) on desktop or any other place -then open TB, open your profile again and now delete (same file(s)) prefs.js now. Restart TB.

Do you see any difference?

I'm not sure that I quite understand the instructions, because if I delete prefs.js then I lose all of my email accounts and have to reset up again. However, I have just tried with a completely new profile, and set up just one email account in it, added MailMerge to my new profile, and get exactly the same problem.

I have downloaded and re-installed Thunderbird 68. Of course, I can't use this with my existing profiles because they have been updated to TB 78. However, created a new profile, installed MailMerge, works as normal. (It also has MailMerge as an option under Send, which I much prefer and seems to have gone from TB78)

The copy action mentioned above was for "back-up". Also, if you delete those files while TB is on (active/working) then when you'll close TB that file will be created automatically again... It was safe...

Anyway, now, for downgrade you can still use your old profile if you want... First start with command -p to select from profile manager that "not working anymore" profile and start it. It will show an error - click exit. Now start again with a command: --allow-downgrade

Thanks again for your help. I tried your suggestion of deleting prefs.js (I made a copy first, of course), while TB is running, and next time I started it did indeed create (or re-create) prefs.js. However, this didn't solve the problem. I don't want to downgrade the profiles because I want to remain on the release update channel, but I have two email accounts that I regularly use for MailMerge, so I will make two profiles that I only use when I need MailMerge and run these from TB 68 if necessary. By this I mean that I have renamed TB 78 to Thunderbird78.app in Applications (this is MacOS), downloaded TB 68 from https://download.mozilla.org/?product=thunderbird-68.1.1-SSL&os=osx&lang=en-US (yes, I know this is an old version - I allowed it to update to 6812.0 but NOT to 78.2.2.), installed it in Applications, and then renamed this downloaded/updated version to "Thunderbird68.app". I've tried completely new Profiles, even a clean profiles.ini file, nothing seems to make MailMerge work with TB 78. I also usually put profiles in a non-standard folder so that I can keep them separated for each project I work on. However, it might be a permissions problem since I never run with Admin privileges maybe? I'm surprised, if there is a bug with MailMerge on TB78, if nobody else has this problem?

However, downgrading to TB 68 is only part of the answer (and it's not a satisfactory solution, but it's a work-around), because you have to stop it automatically updating to TB 78! I had to disconnect from the Internet, turn off WiFi, and then to Preferences -> Advanced -> Update to turn off automatic updates.

As MailMerge is an add-on you might try contacting the authors and report them this issue. They might have also a solution as, indeed, downgrading TB should not be considered as a permanent solution...

How do I find the Author(s) to contact them?

From the Add-On details web page, which you can see from Tools -> Add-ons-> Extensions then click on MailMerge in "Manage Extensions"

It says: "Thunderbird 78: Mail Merge 7.2.1 is fully compatible with Thunderbird 78!

Variable(s) in the To field in Thunderbird 78: You must hit "Enter" or "Tab" once you have added the variable(s) in the To field. This change is caused by the new "pills" in Thunderbird 78."

So that's the answer - RTFM (although admittedly this is quite obscure in this case).

I've tried this - it works, thank you! Apart from this, this is a fantastic and very useful add-on.

(Although now I see that the Subject is not appearing for some reason?)

"Enter" or "Tab" (but not simply moving the cursor to the next field) solved my problem with entries in the "To" field in the header.

I noticed that for sending email from some accounts (but not others - I have lots of email accounts), the Subject line was removed at the moment of saving the message to the Templates folder.

It turned out that this is something to do with end-to-end encryption. It only happens for email addresses which have a personal OpenPGP key (and maybe S/MIME, but I haven't tried). I found that I can "fix" this by selecting not to use OpenPGP for this identity: Tools -> Accounts -> account -> End-To-End Encryption -> "Thunderbird found 1 personal OpenPGP key associated with xxx" -> click "None" Do not use OpenPGP for this identity.

For me, this is a very minor problem, but there could be others, and maybe myself in other circumstances, who would like to digitally sign Mail Merge emails.