This should be a feature! Rated 5 out of 5 stars

Great! Exactly what I was looking for, now I can't close my calender accidentally!

Right click on your tab, and select: "Protect Tab" and it's locked - cannot be closed unless you right-click and un-select it. This should be a feature in the next version of Thunderbird.

5/5 stars!

This user has a previous review of this add-on.