Disable tap opening animation in browser_drag.js to make synthesizeDrop work

browser/base/content/test/browser_drag.js

@@ -28,7 +28,7 @@ function test()
   EventUtils.synthesizeKey("VK_ESCAPE", {}, window);
 
   // now test dragging onto a tab
-  var tab = gBrowser.addTab();
+  var tab = gBrowser.addTab("about:blank", {skipAnimation: true});
   var browser = gBrowser.getBrowserForTab(tab);
 
   browser.addEventListener("load", function () {