http://ted.mielczarek.org/code/mozilla/bookmarklet.html

javascript:(function(){var script=document.createElement('script');script.src='https://cdn.bootcss.com/jquery/3.2.1/jquery.min.js';document.body.appendChild(script);const event=document.createEvent('HTMLEvents');event.initEvent('input',false,true);document.querySelector("#username").value="admin";document.querySelector("#username").dispatchEvent(event);document.querySelector("#password").value="password";document.querySelector("#password").dispatchEvent(event);setTimeout(function(){document.querySelector('button[type=submit]').click();},1000);})();