Well Microsoft kind of owns the world. I've used both IE and Firefox and for some reason I just didn't like the way Firefox constructs the page when downloading the file from the server. And honestly programming specifically for Firefox would be kind of a blunder since (last time I checked) 80 percent of the internet audience uses IE.
That being said, it is common knowledge that IE is weak in it's spec interpretation of internet protocols. But so far, I have not seen any effort from Microsoft to correct these issues. As far as getting Outlook Express to use Firefox as the default web browser, I would be real surprises to see that happen. You would probably be better off finding a third party mail reader that had the ability to change browser defaults within the program. Seems to me a tried several mail readers when I was considering converting from Netscape to IE, but I don't know if they still exist. It's pretty darn tough for a company to compete against someone giving away a product.
