Do you want use FireFox as your default browser but Outlook won't use it when you click on hyperlinks? First, make sure Firefox is set as the default browser on it's Tools, Options menu.

If it is set as the default and either two browser windows open or a File Open dialog opens along with a browser window, then you need to remove the check from Use DDE on the HTTP protocol.

Open Windows Explorer's Tools, Folder Options, File types tab and locate the HTTP protocol, labeled " URL:HyperText Transfer Protocol'. Select it, then the Advanced button. Select 'open' and Edit. Remove the check from the Use DDE box and exit back to the File types dialog. Repeat for HTTPS and FTP protocols.

For more information, see http://www.slipstick.com/problems/firefox.htm