When the Write window is open is the contact Sidebar showing? If not press F9 to turn it on. Hold the control key while clicking multiple contacts. When they are all selected then click Add To.

Yes, the contact sidebar is showing. I tried holding the control key and selecting multiple contacts, but only the first contact was entered in the "TO' list.

I have no issue adding 1 or 21 to a message if tey are highlighted when I click Add to To: so restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.

OK I figured out the problem. When I imported the contacts, all Emails were entered as an alternate e-mail. (Not sure why) I have to move them to the primary e-mail before the selection works. Tedious for the full list but doable. Would there be a short cut to do this?

Irvine_Ray said

OK I figured out the problem. When I imported the contacts, all Emails were entered as an alternate e-mail. (Not sure why) I have to move them to the primary e-mail before the selection works. Tedious for the full list but doable. Would there be a short cut to do this?

It is probably easier to re-import them, this time pay extra attention to the field maps that the CSV import uses.