When you open a ICS file with Thunderbird (by clicking on it and asking you OS to open it with Thunderbird), it shows a wizard to import the events. When you click on the button to import them, it imports them and then shows another window with the import results. You have to click another button to exit the from this import window.
Since most of the time the import is successful, it would be good to have an option to avoid that additional step when there are no errors (in order to streamline the import process). I imported roughly 1000 events this year, all successfully, and this option would have saved me 1000 clicks!