They are not well documented and tend to vary with platform, version and theme. In short, those blue and purple ones indicate replied or forwarded messages. There is a somewhat dated list here: http://kb.mozillazine.org/Icons_-_Thunderbird_2 Even if that doesn't match exactly what you have, it should give an idea of the scope. There is an assumption that these are all self-explanatory and intuitive, and hence don't need any documentation. ;-)