I updated my response for the comments where the users might have asked for left double click on the tab header itself.
Apparently that is not supported in the new WebExtensions architecture used by Firefox.
Even otherwise, the main difference between using double-click-on-tab-header (other extensions) vs double-right-click-anywhere-in-the-page (this extension) is that in this extension, the user doesn't need to move the mouse at all when closing the current page :-)
