Like some other desktop environments, GNOME comes with a set of pre-configured defaults to open files and launch applications, depending on the type of file. For instance, URLs will open by default in a Web browser, emails will open in a mail client, text files will open with a text editor. Depending on the applications installed and what distribution you are using, these defaults may vary and, more importantly, they may not be the defaults that you want.
Configuring default applications and associations in GNOME is easy. To change the global default application, go to System | Preferences | Preferred Applications. Here, you can set your preferred default browser, mail reader, multimedia player, various accessibility programs, and the default terminal application that you want to use. So if you prefer Thunderbird to the GNOME default of Evolution, you would make the change here.