If the add-on does not work for you, the reason is probably, that YouTube uses JavaScripts to reload only parts of the page if you click directly on a video link. As the add-on requires the page to be reloaded by Firefox to detect a /shorts/ link, it does not work. You can "enforce" the add-on to work by then reloading the shorts page (F5 or Ctrl+R). By clicking "open in new tab", also Firefox and not the JavaScripts of YouTube handle the page loading, so the add-on can intercept it as expected.