When you consider Windows Live Mail, it has a sleek, crisp and clean user interface, wherein the contacts list opens in its own window. Some users find it handy, while some of them find it rather annoying or probably next to useless (if not annoying).
On the other hand, its counterpart Thunderbird comes with a user-friendly interface and the basic functionalities. However, there are a number of extensions and plug-ins available from the house of Mozilla, to make it more powerful.
When the discussion boils down to a comparative analysis of Thunderbird versus Windows Live Mail, in terms of extensibility, the latter one turns out to be a big loser. With the handy Contacts Sidebar extension, Thunderbird can display contacts in the main window, while no such option is available for WLM.