User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/83.0.4103.116 Safari/537.36 Edg/83.0.478.64

Steps to reproduce:

Installed update to Thunderbird (Win 32 bit) to version 78.0.

Actual results:

Provider for Google Calendar - a critical extension function to many users - fails where there is no update to this extension available. Worse, the update to 78.0 also causes the user profile to NO longer be compatible with earlier Thunderbird versions. NO easy method exists to downgrade where creation of a new profile is required and it certainly is not clear what functions and configurations were there originally.

Expected results:

Thunderbird upgrade should clearly warn users of functions, extensions, etc. that will no longer remain after upgrade and allow for opt-out. A non-reversible update due to profile manipulation should also have a clear warning and opt-out capability. Also the upgrade process in this situation should automatically create a local backup prior to changes so that an reasonable downgrade route can be used.

The add-on needs to be updated, nothing Thunderbird core can do about that.
You can access Google Calendar using CalDAV too.

This bug should NOT be closed:

1. Whatever was done to lightning internally also appears to be causing other caldav extensions to fail as well. All of the caldav extension developers should have been notified well BEFORE TB was placed in production.
2. You have NOT addressed any of the warnings that SHOULD have been displayed to end users BEFORE installing this upgarde.
3. You have NOT addressed the need for profile backup(s) PRIOR to making UNRECOVERABLE changes.

We have the same issue.
Because of Google Calendar Provider does not work properly, we can not update to the latest Thunderbird versions.
Updating event in Thunderbird will erase all attendees from event.
Is any way to install older version of Google Calendar Provider in Thunderbird 78. Is there any preferences to allow compatibility with older add-ons.

Thank you

No, but you can use standard CalDAV access instead (if you don't need Tasks).
All you need to do is to subscribe to https://apidata.googleusercontent.com/caldav/v2/EXAMPLE@gmail.com/events

To be clear, when using Magnus' instructions, you will lose Tasks since this is a Google / Provider for Googler Cal specific feature:

1. Go to your calendar tab (CTRL-Shift-C or Events and Tasks > Calendar)
2. Right-click in the area to the left where your calendars are shown and choose New Calendar > On The Network
3. When prompted, enter the full (GMail) username
4. For Location field, enter it in the form of https://apidata.googleusercontent.com/caldav/v2/your-full-gmail-address@gmail.com/events

The rest should be self explanatory. Once you see it's working, you can right-click on and remove (a.k.a. Unsubscribe Calendar) the old one that's using Provider for Google Calendar.