With the recent releases of Thunderbird to the Daily, Earlybird and Beta channels, there are updates for both Lightning and Provider for Google Calendar (gdata-provider) add-ons. You can not at this time update via the Thunderbird add-ons manager, you will need to manually download and install the correct version for both add-ons in order to get your calendar working again in your developmental version of Thunderbird. Thunderbird 7 users are able to use the most up to date version of these add-ons which should have already been updated.
For those using Thunderbird 8 Beta, Thunderbird 9 Earlybird or Thunderbird 10 Daily builds, you will to go here to start. Just click on the version of Thunderbird you are running. This will take you to the FTP site. Select your OS, then just choose the most recent build. Be sure to download both files via right-click and save link as… (you may be able to drag the add-ons into the add-on manager, but that was not working for me earlier). Once you’ve download the files, go into Thunderbird’s Add-on Manager and click on the Gear Button (Menu) and select Install Add-on From File. Restart Thunderbird and your calendar should be working fine.