Google provides users with a bunch of services to users which include email, contacts, calendar, news, notebook and more. Accessing each of these services would require you to open multiple sites or tabs to access each of the service.
But like Google Talk you can access all the Google services in your Firefox sidebar using the mobile versions. Amit has put up list of all the services you can access in Firefox sidebar and the URL you will need to bookmark in order to access it.
For example to access Gmail in the Firefox sidebar you can bookmark http://m.gmail.com and choose to view it in the Firefox sidebar.
If you are looking to add more Google Services to your Firefox sidebar check out how you can Access Google services in your Firefox Sidebar.