Backing out string removal from bug 597178 so localizers don't get confused. a=backout

toolkit/locales/en-US/chrome/mozapps/extensions/extensions.dtd

@@ -11,6 +11,9 @@
 <!ENTITY installAddonFromFile.accesskey       "I">
 <!ENTITY toolsMenu.tooltip                    "Tools for all add-ons">
 
+<!ENTITY cmd.back.tooltip                     "Go back one page">
+<!ENTITY cmd.forward.tooltip                  "Go forward one page">
+
 <!-- global warnings -->
 <!ENTITY warning.safemode.label                    "All add-ons have been disabled by safe mode.">
 <!ENTITY warning.checkcompatibility.label          "Add-on compatibility checking is disabled. You may have incompatible add-ons.">