Change Loading.mng to a GIF.

toolkit/skin/win/browser.css

@@ -28,7 +28,7 @@ tabpanels {
 }
 
 tab[busy] > .tab-icon {
-  list-style-image: url("chrome://global/skin/icons/Loading.mng");
+  list-style-image: url("chrome://global/skin/icons/Loading.gif");
 }
 
 tab[selected="true"] {

toolkit/skin/win/jar.mn

@@ -53,7 +53,7 @@ classic.jar:
 +  skin/classic/global/checkbox/cbox-check.gif                    (checkbox/cbox-check.gif)
 +  skin/classic/global/icons/Error.png                            (icons/Error.png)
 +  skin/classic/global/icons/folder-item.png                      (icons/folder-item.png)
-+  skin/classic/global/icons/Loading.mng                      (icons/Loading.mng)
++  skin/classic/global/icons/Loading.gif                      (icons/Loading.gif)
 +  skin/classic/global/icons/Message.png                          (icons/Message.png)
 +  skin/classic/global/icons/Question.png                         (icons/Question.png)
 +  skin/classic/global/icons/Warning.png                          (icons/Warning.png)