To diagnose problems with Thunderbird, try the following:

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.

The whole problem is rather confusing. The first time I saw the red address the mail would not send (invalid address) Now they usually send whether they are black or red. Also I have now seen the problem with a Maverick OS. I have also found a reference to a bug in an earlier version of Thunderbird, that caused a red address in certain circumstances. A bit of a mystery. I presume the address should not print in red.

red in normal terms simply signifies the address is not in your address book.

So perhaps there is one issue, or possible two.

What address books do you have when you open the address book window?

Kontacts and Mac OS address book

No "Personal Address Book" or "Collected Addresses"?

I am sorry to be a bit dim but all I have is a list of contacts entered in Thunderbird. I never have understood the difference between contacts, and address book entries. Also just to be more confusing now Thunderbird is sending emails with both red and black addresses and I cannot find any logic. Usually a typed entry will be red but not always. Entries from the contact list can be both red and black. Answered emails can be both. No logic. At present I seem to be bale to send emails ok. Lets hope it remains that way.