Bug 613767 - Return in tab modal prompts doesn't trigger default action [r=dolske a=dolske]

Pass the number instead of the xul:button.

--HG--
extra : rebase_source : 4ef8ddbf765efe40ccb6e4e389112926cef82749

toolkit/components/prompts/content/tabprompts.xml

@@ -208,7 +208,7 @@
                     let button = this.ui["button" + bnum];
                     if (!button.hasAttribute("default"))
                         return;
-                    this.onButtonClick(button);
+                    this.onButtonClick(bnum);
                 } else { // action == "cancel"
                     this.onButtonClick(1); // Cancel button
                 }