Detail information on how it freeze is descripted in attachment. On average, it will freeze after 7~12 events is created. Restarting thunderbird will resolve it, but it will happen again. After restart, the changes that is done during freeze can be seen. For example, If I created a event call "Event A" during freeze, I can only see it after restarting thunderbird. I use TbSync to sync my iCloud calendar. I tried save mode which disable all add-on, but nothing changes. pls help.