If you highlight a person's entry in your addressbook, and then click edit, look for where the multiple email addresses appear and there is a column to the right where you can tick the default.

Thanks for your response.

So, done that...I can't remember that I've found this setting in the past when I've read posts about this issue. Anyway:

• the contact shows the "desired" default address at the top when I set the default (in the view where I just select the contact)
• the hook for default is not shown when editing the contact again
• if I select another of the five addresses, it's quickly shown but then changes back to the desired address, which is the top address in the list of five
• anyway, when composing a new message, the chosen address is still the one from before

Now I've added a test contact, and which ever of two email addresses is at the top is chosen by TB for composing an email. Unfortunately this is not the case with some other users. The only way getting rid of the wrong default is by removing the email address, as soon as I add it, it is #1 again.

Could it play a role that the contacts are stored in GMail and imported via CardDAV URL?

In my brief test, the default box is not saved for gmail addressbook - only local addressbook. But it works properly for local thunderbird addressbook.