You'd need to be more specific about your problem. A screenshot may also help. https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

Is this from sending through an ATT/Yahoo account?

Thanks for your reply. I have several email accounts which are all automatically forwarded to Thunderbird. My outgoing messages are indeed sent from Thunderbird through one of my Yahoo accounts. What I don't understand is that this only started happening a few days ago yet I've had the same setup (Thunderbird-Yahoo) for several years now.

There was a recent change at Yahoo that caused the issue. The workaround is explained here, although some users report that Yahoo has fixed it.