Comment 29 for bug 429395

Ok, I got the information from Ruben's (?) blog (ubublogger.wordpress.com) how to add a starter entry in the indicator applet.

"[...] launch Thunderbird from the Indicator Applet, you can create a file named ‘thunderbird’ in the /usr/share/indicators/messages/applications/ folder with the following contents: ‘/usr/share/applications/thunderbird.desktop’ "

In my case I had to use /usr/share/applications/thunderbird-3.0.desktop