This will most likely work, depending if the Thunderbird installation is in your path. If not, then you need to find where Thunderbird is installed and use the full directory. For example,
Ubuntu comes with alacarte menu editor, I think, I don't use Ubuntu. I am assuming your are using gnome, since you didn't say, you can add menu items with ease using alacarte. To Add an icon to a panel, you either right click on a menu item and choose to add it to a panel, or you can right click on a panel and choose "add to panel"
You will get a new window asking what you want to add. You can either choose to use an application from a menu or a custom application launcher. For the command, just type: thunderbird or the complete directory.