Send button is gray-out if no email is at TO field or the email is not complete or if you enter an email address but do not click enter/tab (w/o enter/tab Thunderbird don't consider that as a valid email). Far as I could test if you click on tab it confirm the email address and next tab goes to subject... is not the same on your side?

Unfortunately the address is valid and I can't press enter/tab in the To: field anymore. It is possible to jump from subject to message body with pressing tab though.

When it comes to the issue with the address book issue there are a lot of exceptions in the error log (ctrl shift j in my thunderbird version). They look similar to this line:

[Exception... "Component returned failure code: 0x80630002 (NS_ERROR_STORAGE_IOERR) [mozIStorageService.openDatabase]" nsresult: "0x80630002 (NS_ERROR_STORAGE_IOERR)" location: "JS frame :: resource:///modules/AddrBookDirectory.jsm :: openConnectionTo :: line 78" data: no]

AddrBookDirectory.jsm:78:35

The same is true for the CC / BCC fields. It is not possible to press TAB/ENTER in them anymore. Keypress gets ignored.

Ref to/cc - a message wrote by another user: Thunderbird 78 has changed compared to Thunderbird 68, that's why you see this discrepancy. In 68, the field name is a dropdown list, and you can change To to CC/BCC. In 78, however, you can add CCs and BCCs by clicking on the text buttons beside the To field.

Yes but why is the Send button grayed out and why can't I use the Tabulator / Enter KEY on my keyboard anymore when I am in the To/CC/BCC field?