User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; de; rv:1.8.1.3) Gecko/20070309 Firefox/2.0.0.3 Build Identifier: Version 2.0.0.0 (20070326) Right now there is no easy way to install thunderbird add-ons. If you click on any install now buttons in a.m.o, Firefox wants to install them. There should be a way to just click on the install now button and firefox sends the .xpi to thunderbird and it gets installed there. Alternatively, thunderbird could show a.m.o. in a thunderbird window and allow a direct installation from there. Reproducible: Always Steps to Reproduce: 1. 2. 3.