Many times there are features that don't make it into the milestone releases or may be planned for some time down the road. Thunderbird's extension support allows developers to create extensions that will plug into Thunderbird to extend its already powerful capabilities. An example is an extension, such as Allow Empty Subject, which basically allows you to bypass the warning dialog box when you send a message with an empty subject (obviously someone felt passionate enough about this feature or creating extensions in general to spend the time to design such an extension). There are extensions that run as applications within Thunderbird, such as Mozilla Calendar and Forumzilla. There are even extensions that mimic behavior in Outlook, such as the Purge extension. If you look hard enough, there is probably an extension out there that you can use to enhance your use of Thunderbird.