First of all, try to open the ''doc'' and ''docx'' files from your file manager and see what opens them. If it's not to your liking, you need to define the system default program. Or you may find that shift-right-click gives you an option to set the preferred application. By and large, if Thunderbird cannot or does not know how to open a given type of file, it will just hand it over to the OS to deal with it. That's why setting the system defaults is useful. In Thunderbird, go to '''[http://kb.mozillazine.org/Menu_differences_in_Windows%2C_Linux%2C_and_Mac Tools|Options]|Attachments→Incoming'''. If ''doc'' and ''docx'' files are listed there, then you can set the preferred application. The default "Use default" setting means that the system default program should be used. The frustrating thing with this settings panel is that if your file type doesn't appear, there's no easy way to add it so as to allow you to give Thunderbird particular instructions.