Rated 4 out of 5 stars

This does exactly what I need it to do: Allow me to create a custom one-time from: field.

A link below in one of the other reviews has a version that works with 2.0+. I tried that version and it works. The only reason I didn't give it 5 stars is that it's apparently no longer maintained.

A link in one of the other reviews (see link to view all reviews) gave me version 0.3.8 that works with thunderbird 2.0+:
http://forum.addonsmirror.net/ext/extthunderbird/TB_Change_From_and_Fcc_on_Compose_Extension_0.3.8.xpi

But the author is apparently no longer maintaining any extensions:
http://forums.mozillazine.org/viewtopic.php?f=28&t=585171