In Firefox 3 it is easy to edit the MIME settings. These settings are used to control what program is used by default to open each file type you encounter while browsing.
To edit these settings in Firefox 3 navigate these menus:
Edit -> Preferences -> Applications
For each file type on the left you can designate a program to open that file type on the right. You can also avoid opening a specific file type and choose to save it to disk by default instead.