The thread is marked SOLVED. Have you found a solution?
You could create a launcher as described here
If GNOME 2 is freedesktop.org compliant, you should be able to add a menu item for the ~/Desktop installation of Thunderbird by adding a .desktop file for it in ~/.local/share/applications.
Here's an untested and cobbled together example of thunderbird.desktop in case one was not installed that you can copy. You would need to change at least the Exec= and Icon= lines and may want to change the Categories line to suit the GNOME 2 menu.
Exec=/home/<your user name>/Desktop/<whatever>/thunderbird
Icon=/home/<your user name>/Desktop/<whatever>/thunderbird.png
BTW why did you install Thunderbird under ~/Desktop rather than in the conventional place (in which case it probably would have shown up in your menu)?