Thunderbird 15 has been released and with that is an update to Lightning to the new 1.7 version which was released a couple days ago. I restarted Thunderbird after the updates and while I had my tasks, I had no calendars. I looked at the add-ons manager and saw I was running Provider for Google Calendar version 0.13 which is the most current release version. I checked under developmental versions and found a link to download the 0.16pre version. Installed this version, restarted and I had my Google Calendars back in Thunderbird. I am not sure if it is the new Firefox 15, but for some reason clicking on the Download for Windows links over on AMO tries to install the add-on into Firefox instead of downloading. Right-clicking and selecting Save Link As… will get you around this little annoyance.