Sorry I never got back to this ... but applying this "solution" allowed this to work as I want only once. Then everything reverted to opening My IE.

I since tried un-installing My IE and the re-installing FireFox but then NO browswer opened when I double clicked a link in Outlook. It seems I HAVE to have My IE installed to open any links in Outlook.

Any other suggestions? I'd hate to have to re-install XP HOME and all my software (minus My IE) just to have this feature work correctly.