Mozilla Thunderbird 1.5 users may have noticed that when you start typing an email address in the mail composition window, Thunderbird no longer automatically adds the default domain to it. You can turn this feature back on, if you’d like. Go to Tools -> Options -> Advanced -> General, and click on “Config Editor.” In the resulting window, search for the setting mail.identity.default.autocompleteToMyDomain. Double-click on it. That should change the value to true. Close the Config Editor, and click on “OK” in the Options panel, and you’re done.