When you get a new desktop notification, your browser will ask for your permission to show the desktop notification by default.

Chrome:

Click the "Allow" button in the pop up

Firefox:

Select "Always Show Notifications"

You can always change the notification permissions for a site.

Chrome:

1. go to Settings
2. click the "Show advanced settings" link at bottom
3. click the "Content settings" button in Privacy section
4. scroll down to "Notification" section
5. click the "Manage exceptions..." button

Firefox:

1. Open the site