I got it figured out....kinda.
I uninstalled Avant and the next link I clicked in Thunderbird opened IE. The next time I opened FireFox it asked if I wanted it to be my default browser. Now when I click a link in Thunderbird, FireFox opens.
All's well that ends well.
The simple believeth every word: but the prudent man looketh well to his going. ~Proverbs 14:15