Integrating Firefox and Outlook Express
From MozillaZine Knowledge Base
Opening Firefox from Outlook Express
When clicking on a link in Outlook Express, Windows will open the default browser. There is no known way of making Outlook Express open links in a different browser. Hence to make Outlook Express open Firefox, you must make Firefox the default browser.
ZoneAlarm may cause Firefox to not respond to links opened from Outlook Express. Try disabling or uninstalling ZoneAlarm to see if it solves the problem.
Opening Outlook Express from Firefox
When clicking on a mailto: link in Firefox, Windows will open the default mail client. To make Firefox open Outlook Express, you simply need to set Outlook Express as your default mail program.