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.
Well, if crime fighters fight crime and fire fighters fight fire, what do freedom fighters fight? They never mention that part to us, do they?