diff --git a/toolkit/components/prompts/content/tabprompts.xml b/toolkit/components/prompts/content/tabprompts.xml
index 07c6c8efb..0ce13203c 100644
--- a/toolkit/components/prompts/content/tabprompts.xml
+++ b/toolkit/components/prompts/content/tabprompts.xml
@@ -323,21 +323,19 @@
             let bnum = this.args.defaultButtonNum || 0;
             let defaultButton = this.ui["button" + bnum];
 
-            let { AppConstants } =
-                Components.utils.import("resource://gre/modules/AppConstants.jsm", {});
-            if (AppConstants.platform == "macosx") {
-              // On OS X, the default button always stays marked as such (until
-              // the entire prompt blurs).
-              defaultButton.setAttribute("default", true);
-            } else {
-              // On other platforms, the default button is only marked as such
-              // when no other button has focus. XUL buttons on not-OSX will
-              // react to pressing enter as a command, so you can't trigger the
-              // default without tabbing to it or something that isn't a button.
-              let focusedDefault = (event.originalTarget == defaultButton);
-              let someButtonFocused = event.originalTarget instanceof Ci.nsIDOMXULButtonElement;
-              defaultButton.setAttribute("default", focusedDefault || !someButtonFocused);
-            }
+#ifdef XP_MACOSX
+            // On OS X, the default button always stays marked as such (until
+            // the entire prompt blurs).
+            defaultButton.setAttribute("default", true);
+#else
+            // On other platforms, the default button is only marked as such
+            // when no other button has focus. XUL buttons on not-OSX will
+            // react to pressing enter as a command, so you can't trigger the
+            // default without tabbing to it or something that isn't a button.
+            let focusedDefault = (event.originalTarget == defaultButton);
+            let someButtonFocused = event.originalTarget instanceof Ci.nsIDOMXULButtonElement;
+            defaultButton.setAttribute("default", focusedDefault || !someButtonFocused);
+#endif
         </handler>
         <handler event="blur">
             // If focus shifted to somewhere else in the browser, don't make