Fix link opening from the add-on manager window, r=clokep.

im/content/extensions.js

@@ -51,6 +51,21 @@ var addonsRegister = {
 
     window.removeEventListener("load", addonsRegister.onload, false);
     window.addEventListener("unload", addonsRegister.onunload, false);
+
+    let win = document.getElementById("dummychromebrowser").contentWindow;
+    let open = win.open;
+    win.open = function(aUrl) {
+      let uri = Services.io.newURI(aUrl, null, null);
+
+      // http and https are the only schemes that are exposed even
+      // though we don't handle them internally.
+      if (!uri.schemeIs("http") && !uri.schemeIs("https"))
+        open.apply(this, arguments);
+      else {
+        Cc["@mozilla.org/uriloader/external-protocol-service;1"]
+          .getService(Ci.nsIExternalProtocolService).loadUrl(uri);
+      }
+    };
   },
 
   onunload: function () {