When an address is red, it means it's not in an address book, and the Send button is not active (first picture) until the address is 'accepted' by clicking on another field such as Subject (second picture). But in your first picture, the address is still red and not shown in a 'pill' even after the focus (cursor) is in another field, the Subject.

Some add-ons like Card Book and Signature Switch can cause such problems, so I suggest you run in safe mode (hold Shift when launching TB) to check for incompatible add-ons.

I have run Thunderbird in safe mode, but the result is still the same (send button is not active).

I also have opened the address book, which is empty. Is this normal?

Leopold Coninx

If you had contacts in the address book before the upgrade to 78, then they should have been retained. That is a separate problem, which shouldn't affect the issue with the Send button. The problem seems to be related to the fact that the recipient contact isn't enclosed in a pill after you move the focus to another box.

Was this on a new setup of TB 78 or an upgrade from 68? You could try creating a new profile, adding an account, and see if the problem remains. Help/Troubleshooting, about:profiles, to create and manage profiles. Your original profile is unaffected, and if the new profile works properly, data can be transferred from the old profile to the new.