I was using Thunderbird 91.x and was using TbSync and Provider for Exchange ActiveSync extensions to update my calendars, etc.
When I did sudo apt update and sudo apt upgrade, my Thunderbird was upgraded from 91 to 102. I noticed that my calendar was not being updated to my phone and doing some investigation, I found that TbSync and Provider for Exchange ActiveSync are not (yet?) supported on 102. I am lucky because all of my emails are IMAP and I don’t create any local folders to store data. Therefore, I had to find and install Thunderbird 91, reset all of my environment (email accounts, preferences, extensions, etc) and now it all works again.
I put this note in so that someone else might not have to go through the same issue.