Yes, the only way to block a window from closing is to have one of your tabs raise an alarm when you try to close it. You don't need it in every window because Firefox can restore most closed windows from the History menu, but you do need it in every private window because Firefox cannot restore those.

An alternative design would be to inject that script into a tab *you* open in the window. But then it would need to be re-injected if you left that page or closed that tab, so it seemed safest to have a dedicated tab.

I can look at the new tab blocking issue. That is because protection needs to be activated manually for each window you want to protect. Maybe it is possible to do that automatically and then slide the tab out of the way. But there would still be a tab.

If you find a better solution, please update your review to let me and other users know what it is.