offsite-links-new-tab.user.js

// ==UserScript==
// @name        offsite links new tab
// @namespace   fent
// @version     0.1.1
// @description Opens offsite links in a new tab.
// @include		  /https?:\/\//
// ==/UserScript==

var originalHost = window.location.host;
document.body.addEventListener('click', function(e) {
  var target = e.target;
  if (target.tagName === 'A' && target.target !== '_blank' &&
      (target.protocol === 'https:' || target.protocol === 'http:') &&
      target.host !== originalHost) {
    // Setting this property instead of using `GM_openInTab()` works
    // better for links that are handled via Javascript.
    // If `e.preventDefault()` is called on this event, a tab won't
    // be opened incorrectly.
    var oldTarget = target.target;
    target.setAttribute('target', '_blank');
    setTimeout(function() {
      target.setAttribute('target', oldTarget);
    });
  }
});