The first time you run Thunderbird, it should ask you if you want to make it your default mail client. If you answered no and later decide you would like it to act as your default mail client:

• Windows, Mac: go to Tools > Options..., click on General and check Mail, then click OK. If you still have problems with mailto: links under Windows, go to Start > Control Panel and open Internet Options. Under the Programs tab, select Mozilla Thunderbird from the drop-down list next to E-mail:.
• Gnome (Unix/Linux): In the Gnome main menu, go to System > Preferences > Preferred Applications and then under the Mail Reader option, select Mozilla Thunderbird.
• KDE 3.x (Unix/Linux): Open the Control Center and go to Network > Email. Under Preferred Email Client, enter: absolute path of Thunderbird install directory/mozilla --mail