I used to use the Gmail Provider addon for my Google Calendar, but it had been acting funny for months when it came to syncing. So I switched to native support and everything was working great mostly. But then the last few days or so, the Calendar stopped showing some of the events. Some of them show just fine but a lot of my events are missing. They are still there on the Gmail side of things. They just don't show in Thunderbird. I found a recent chat about this exact thing but it was an outdated issue that I believe was fixed. But here it is again (or maybe a different cause/bug is behind it this time). I don't know if its the difference between events that show you as busy or free, or if something else is the variable the determines what shows and what isn't. But it is definitely not working right now. I had to sync my Google calendar with with Windows Calendar for the time being.