Rated 5 out of 5 stars
For those having problems with getting this working with the newer versions of Thunderbird and Firefox I would like to point out there is another extension biased of this one which is actively being developed by me. See https://addons.mozilla.org/en-US/firefox/addon/2831
I only take the liberty of posting this review because one of the original developers emailed be recently with this to say (only part of a longer email):
"I've heard from others that another extension existed. Sure enough, I saw it did. Thank you very, very much for taking over this concept. It was needed desperately when Mook and I worked on it. And it's still needed now. What you've done is wonderful. Great work!"