It will close the current tab if you double click anywhere on the page or the tab. Also, if you right click on any page, you will get the "Undo Closed Tab" context menu option to open again the last tab or tabs if you closed it by mistake or changed your mind.
Why this new Double Click to Close Tab addon?
Similar addons does exist, they are bigger, they do not always react to the double click action and to have the "Undo Last Tab" option you need to install yet another addon too. I know it because I have struggled/tried them all before doing this addon!
How big is it? 1Kb!! It is the smallest/fastest addon ever made.