Does the same thing happen in Safe Mode?

If so, an incompatible add-on could cause it, maybe one related to contacts, but not necessarily. Restart Thunderbird for normal use, disable all add-ons and re-enable them one by one to find the one responsible for the issue. Try to find an updated version or leave the add-on disabled until one is released.

Does this help?

Sorry even with add-ons disabled the problem continues.

Do any of these work?

drag and drop "add to..." button right-click

Double click appears to me to be the one non-obvious, undocumented method, so I'm curious as to if any of the explicit methods work.

Sorry as soon as I click on any name in the contact list or anything else the box next to the " To:" in the email being "forward" goes back to grey as if it is shutting down and not prepared to accept anything.

That sounds to me as if maybe the address has been added and has scrolled up to make way for the next.

Try to grab and drag the horizontal divider between the addressing space and the message text box. Pull it down to make more room for addresses.

There really ought to be a scrollbar in this situation, but I have just noticed that whilst I can get one, it takes a resizing exercise to make it appear.

But do try running Thunderbird in its Safe Mode, to check if you have any add-ons that are interfering with normal operation.

https://support.mozilla.org/en-US/kb/safe-mode-thunderbird

Would you believe that this is a new computer and they set up my email for me to save me some time and rather than placing the email address in the slot for "emails" in the contact list they put it in the "Last Name" slot. Of course it will not transfer as there is NO email. Sorry for the problem but you know what they say when you "assume" something was done correctly without checking it carefully. PROBLEM solution found.

Thanks for reporting back. Could you mark your last message as a solution?