Mozilla released the first Beta for the upcoming Thunderbird 20 on March 8th, 2013. Lightning Users: if you are using Google Calendar with Lightning, you should wait on this update. Either Thunderbird 20 Beta 1 or Lightning 2.2beta1 seems to break the Google Calendar integration (even with version 0.18 of Provider for Google Calendar extension). Users will be continuously prompted to enter their Google Calendar (gmail) password, which suggests there is an authentication or communication issue between Thunderbird and the Google Calendar server.
Fixes in Thunderbird 20 Beta 1 include Emails can now be sent to IDN based email addresses (bug 127399) along with various fixes and performance improvements. Thunderbird 20 (numbered as 17.0.5) is scheduled for release on April 2nd, 2013.