Create a mailing list. http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Create_Mailing_List

Many thanks, but doesn't fully solve problem. It's a one-off collective mailing. Maybe no alternative though?

It's a one-off collective mailing. Maybe no alternative though?

I'm not sure what that means and what else you're looking for.

If I add , say, 80 addresses, it won't send (says 50 max.). Perhaps I must just count them as I add them and stop at 50?

I'm not going to need to send to the same 80 again, so a formal list doesn't seem necessary.

Your email provider may limit the number of recipients. Talk to your email provider.

The way to break the built in limit is to use a Mailing List or Mail Merge.

You have the add the addresses to something, either the message itself or a Mailing List. I can't see why either would be significantly more onerous than the other.

Or you just count. ;-)