You can try starting Thunderbird v68 with the --allow-downgrade commandline switch to downgrade your v78 profile for use with v68. On a v78 fresh install I have though provider add on working.

See my post at https://support.mozilla.org/en-US/questions/1304598. I had the same issue. There is actually a new version of TbSync and Provider for CalDAV and CardDAV that will allow you to do the sync. You need to install the new versions and then setup the accounts again to sync.

What Mark said - you shouldn't need to go back to version 68 ... and that definitely shouldn't be the first choice option anyway.

The automatic updates to 78.* do not deal with the extensions. It may flag some extensions as not working even if there is a new version that will work. Because of the differences in code, many extension authors have two branches, e.g. one version of their extension for older versions and another for the newer ones.

You can use TbSync and Provider for CalDAV and CardDAV as I suggested if you use the latest versions from the extensions website. There is also a newer version of Provider for Google Calendar that works for 78.*. The author for gcontactsync does not certify his latest version for 78.*. Nevertheless, he talks about how he has been working to make the changes needed for the new code. So you may be able to get by fine with just installing the latest versions of both of your current extensions. They will not automatically update, however, as far as I can tell. As I noted, I had to do the updates myself. To be safe, I deleted the old versions, then clean installed the latest versions and did the setups over again.

While it takes a little extra effort to make the manual updates, it makes sense to go ahead so you can be using a current version of Thunderbird instead of sticking with an obsolete version that will eventually stop being updated. You seem to have workable options for the extensions you need at this point.