if (!window.localStorage) {
  window.localStorage = (function () {
    // XXX eliminate dependency on jQuery, detect browsers ourselves
    if ($.browser.msie) { // If we are on IE, which support userData
      var userdata = document.createElement('span'); // could be anything
      userdata.style.behavior = 'url("#default#userData")';
      userdata.id = 'localstorage-polyfill-helper';
      userdata.style.display = 'none';
      document.getElementsByTagName("head")[0].appendChild(userdata);

      var userdataKey = 'localStorage';
      userdata.load(userdataKey);

      return {
        setItem: function (key, val) {
          userdata.setAttribute(key, val);
          userdata.save(userdataKey);
        },

        removeItem: function (key) {
          userdata.removeAttribute(key);
          userdata.save(userdataKey);
        },

        getItem: function (key) {
          userdata.load(userdataKey);
          return userdata.getAttribute(key);
        }
      };
    } else {
      Meteor._debug(
        "You are running a browser with no localStorage or userData "
          + "support. Logging in from one tab will not cause another "
          + "tab to be logged in.");

      return {
        _data: {},

        setItem: function (key, val) {
          this._data[key] = val;
        },
        removeItem: function (key) {
          delete this._data[key];
        },
        getItem: function (key) {
          return this._data[key];
        }
      };
    };
  })();
}

if (!window.localStorage) {
  window.localStorage = (function () {
    // XXX eliminate dependency on jQuery, detect browsers ourselves
    if ($.browser.msie) { // If we are on IE, which support userData
      var userdata = document.createElement('span'); // could be anything
      userdata.style.behavior = 'url("#default#userData")';
      userdata.id = 'localstorage-polyfill-helper';
      userdata.style.display = 'none';
      document.getElementsByTagName("head")[0].appendChild(userdata);

      var userdataKey = 'localStorage';
      userdata.load(userdataKey);

      return {
        setItem: function (key, val) {
          userdata.setAttribute(key, val);
          userdata.save(userdataKey);
        },

        removeItem: function (key) {
          userdata.removeAttribute(key);
          userdata.save(userdataKey);
        },

        getItem: function (key) {
          userdata.load(userdataKey);
          return userdata.getAttribute(key);
        }
      };
    } else {
      Meteor._debug(
        "You are running a browser with no localStorage or userData "
          + "support. Logging in from one tab will not cause another "
          + "tab to be logged in.");

      // XXX This doesn't actually work in Firefox with dom.storage.enabled =
      // false: the assignment to window.localStorage is ignored. If we care at
      // all about this use case, we should probably define Meteor.localStorage
      // instead of doing a polyfill. (This causes this package's test to fail
      // in that situation.)

      return {
        _data: {},

        setItem: function (key, val) {
          this._data[key] = val;
        },
        removeItem: function (key) {
          delete this._data[key];
        },
        getItem: function (key) {
          return this._data[key];
        }
      };
    };
  })();
}

Comment that localstorage-polyfill fails on FF with dom storage explicitly disabled.