Handle a Firefox error relating to websockets.

We've only gotten one error report for this error, and it didn't have a stack
trace, so it's hard to know whether this will actually fix the issue or not.

(imported from commit 7bff052347a33f67d8479b1cb23f6e5e24defc75)

static/js/socket.js

@@ -62,6 +62,12 @@ Socket.prototype = {
                 // The connection was somehow closed.  Our on-close handler will
                 // be called imminently and we'll retry this request upon reconnect.
                 return;
+            } else if (e instanceof Error && e.message.indexOf("NS_ERROR_NOT_CONNECTED") !== -1) {
+                // This is a rarely-occurring Firefox error.  I'm not sure
+                // whether our on-close handler will be called, so let's just
+                // call close() explicitly.
+                this._sockjs.close();
+                return;
             } else {
                 throw e;
             }