Dear new version of Firefox,
When you open up a new window, usually against my wishes, and I go to close it, I would appreciate it if only that window would close. You see, what actually happens is I get a pop-up that warns me of all the tabs I will be closing. In fact, I am only closing one – the single tab in the single new window. But you don’t seem to know this. What you seem to think is that the “X” on that new window also counts as the “X” on my main window – the one with a bunch of tabs open. And to make the problem all the worse, when I click “Cancel”, meaning “No, do not close all my tabs”, you go ahead and close everything anyway. Please knock it the hell off.
Sincerely,
Everyone
For the Sake of Science 
It sounds like a bug – report it to Firefox support via bug form instead of reporting it here.
Interesting. I have not seen that behavior. Perhaps it’s because I use linux. I’ll give it a try in Windows.