Nowadays everyone pretty much has 2, 3, or even 4 or more email accounts. In Thunderbird when you click on Write to compose a new email, you can pick via a drop down which email address you want to use to send the email. I started a transition to use Google's Gmail more over Yahoo Mail, so now I want the From field to show my Gmail address as the default when I am writing a new email. Currently my Yahoo email address is the default send from.
To do this I tried going to Tools, and picked Account Settings. All my email accounts showed up, and I see the Yahoo mail settings on top. But there's no obvious way to re-arrange them- assuming that the account showing on top is the highest priority one. Then I noticed a button that says "Account Actions" with a choice that says "Set as Default". So I selected my Gmail account, click on Account Actions, and picked "Set as Default". That immediately put the Gmail account to the top.
Now when I click on Write for a new message, voila, the from field is now showing the GMail address as I wanted. This worked as of Thunderbird version 5.0 so to do this in older versions might involve a different method.