Bug 1527081 - don't create preloaded tabs for session-restored additional tabs, r=dao

Differential Revision: https://phabricator.services.mozilla.com/D19409

--HG--
extra : moz-landing-system : lando

browser/base/content/tabbrowser.xml

@@ -1092,8 +1092,12 @@
           // overflowed.
           this.arrowScrollbox._updateScrollButtonsDisabledState();
 
-          // Preload the next about:newtab if there isn't one already.
-          gBrowser._createPreloadBrowser();
+          // If this browser isn't lazy (indicating it's probably created by
+          // session restore), preload the next about:newtab if we don't
+          // already have a preloaded browser.
+          if (tab.linkedPanel) {
+            gBrowser._createPreloadBrowser();
+          }
         ]]></body>
       </method>