So many of the URL shorteners either 1) don't work with Firefox Quantum (tinyURL, bit.ly, to name a couple) or 2) require several steps to create the short URL. Shorten Me uses goo.gl and it works in one click. You don't need even to select the long URL. As long as you are on its page, just click the button ("S") and it saves the shortened URL to your clipboard. Then just paste. Easy peasy. Very nice.