On at least *some* installations of both the official, and Peter's 
unofficial version of Thunderbird, after installing Lightning, Thunderbird 
will not re-open.  If this should happen to your installation, you can 
easily uninstall Lightning cleanly by right clicking on the Thunderbird 
object, selecting Properties, and entering -safe-mode in the Parameters 
box.  When you restart Thunderbird, go to the menu bar, and click on Tools 
-> Extensions.  Right click on the Lightning extension, and select 
Uninstall.  Shut Thunderbird down, remove -safe-mode from the Parameters 
box, and you can now start Thunderbird as expected.

Good luck,
Sander Nyman