Rated 5 out of 5 stars
Nice Add-on. Have been used to using the Esc key like this since DOS days and find it very helpful.
Like "Click-Click" (in review below) I also experienced an add-on compatibility issue with latest Thunderbird update. I followed the same process that he did in order to resolve things.
If anyone else needs to do this, Google-search for the "How-To Geek" article using the following search terms (complete with quotes) "Hack Extension Files to Make Them Version-Compatible for Firefox". Ignore the "Firefox" references as instructions are the same for Thunderbird.