Changes
Add a remove cmd entry
<upgrade cmd='"%PROGRAMFILES%\Mozilla Thunderbird\thunderbird.exe" -install-global-extension "%PACKAGES%\thunderbird_add-on\lightning-0.3.win32.xpi"'>
</upgrade>
<remove cmd='echo "HOW UNDELETE THIS FROM COMMAND LINE ????%PROGRAMFILES%\Mozilla Thunderbird\uninstall\helper.exe"/S'/>
</package>
</pre>