I think that in addition to the option to close the browser via the "X" button, there should also be an option to close the browser and kill the extensions and apps running in the background all at once.
Believe me, this is necessary many times.
One solution would be to combine a key like SHIFT or CTRL while clicking the "X" button. Another solution would be to show a flyout menu when moving the mouse over the "X" button, thus allowing you to choose the command to close the browser and kill the extensions and apps running in the background.