I am using Windows 10 and Thunderbird 78.6.0. I am not syncing Thunderbird with any external or online calendars. About two months ago, during some Thunderbird update (not this specific version 78.6.0 but back a few, yet the issue continues on with recent updates), Thunderbird's calendar started having bugs. If I left calendar reminders open (either minimized in the Windows taskbar or open on-screen) and I closed the Thunderbird main program window first, then Thunderbird has a high likelihood of deleting the open calendar reminders from my calendar. Prior versions of Thunderbird would simply close the reminders and open them back up the next time Thunderbird opened. Not anymore. There is a high likelihood that Thunderbird will "lose" or "delete" the entire calendar item from the calendar. No reminder will pop up the next time the software is opened because the item has been deleted completely. It is not in the trash either. How do you fix this recent programming bug? The behavioral answer "the user should close all calendar reminders before closing the main program" might work in theory but not in practical use, as it is simply a slip when I close the main program first. It happens and it will happen. But doing so before never deleted reminders! Thank you, Matt