"WebMail 1.4.4 could not be installed because it is not compatible with Firefox 3.6.23"
It is because FF is trying to install the xpi in itself instead Thunderbird. Firefox will always try to install the xpi's in itself as it uses the same extension name for its own add-ons. Right Click on the webmail add-on and download it, then use install from file in the TB's add-on section. Remember where you put the download file. Also if using TB3 see author note on the currently compatible version. Eugene
