If you read at the quicktext site, you'd know that there is an issue with Thunderbird 3.1. It's
NOT Arch's build, it's the plugin and 3.1 that have a problem. Supposedly following these directions will allow you to install the plugin successfully on 3.1
If you are having similar issues, here's the solution.
1. Uninstall old Quicktext
2. Download latest version from this site. There is no need to change anything in the install file as suggested in earlier posts.
3. Install using the procedure outlined here
http://kb.mozillazine.org/Extensions_%28Thunderbird%29