At the top near the search bar, there should be an option to shorten a URL with Firefox, maybe the shortened URL will always be shown on the right of the search bar within a textbox with a copy button. If this feature gets added please make this the shortest domain as you possibly can, and the ID generated after "/" to contain the most character varieties as possible in order for it to be shorter.
NOTE: Make it as soon as the copy button is clicked, that's when the shortened URL officially goes live, this is in order to prevent access to private webpages by constantly visiting random shortened URLs that the end user did not intend to create.