The send button is disabled unless an account to send mail from is selected. That means a valid From: must appear in the compose window above the To: stuff.

If you have something there that does not change between reply and new, I suggest you try restarting 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.

The From line is the same for new e-mails, replies and forwards. I tried safe mode and still no send possible on new or forwards.

Just humor me. What happens if you press Ctrl+ enter instead of clicking send? Ctrl+Shift+Enter?

Have you actually put a valid email address in the To: line?

Click here

Thunderbird will open a pre addressed new email. See if you can send an email to that address. It is one of mine that I use for testing, so you will not be spamming anyone.

I either have found a work-around for the problem or I had something wrong with the To addresses. Still not sure it is working correctly but whatever.