This article describes how to go back to an old version of Thunderbird, including how to deal with problems that may occur due to changes to your profile.
Whatever version of Thunderbird you re-install, it will automatically find and reuse your profile (it contains your mail, address books, settings, saved passwords, add-ons etc.), which is not deleted when you uninstall or reinstall Thunderbird.
You can still add public Google Calendars using the i Calendar (ICS) format, which offers read only access (see below).
Editing the calendar works both ways, meaning you can add and edit events via the website or in Thunderbird and they will automatically sync in either direction.
The Mozilla Wiki has a thorough page covering Provider of Google Calendar, including a section about bugs and limitations.
Let’s see how you can fully integrate Google Calendar with Thunderbird.
Google Calendar integration requires two Thunderbird add-ons: Extensions check whether you still need to install Lightning.