I’m sure it’s much much easier than this now, especially with Ubuntu Linux.
I wrote this a couple of years ago, but should keep it around just in case.
Use a text editor like vi (my favorite) or nano to create a .desktop file for the entry in /usr/share/applications. I named mine thunderbird.desktop. (sudo vi /usr/share/applications/thunderbird.desktop)
Then I added these lines to it:
[Desktop Entry]
Name=Mozilla Thunderbird
Exec=/opt/thunderbird/thunderbird
Icon=/opt/thunderbird/icons/mozicon50.xpm
Terminal=false
Type=Application
Categories=Network
On Ubuntu there are *.xpm files located under /usr/share/pixmap
