Firefox is my main browser and I only use IE if a site requires it.
I do have IE7 - but haven't used Outlook Express for years, so i don't know about any conflicts or problems betwenn the IE 7 & OE.

I use the Thunderbird email program {ffree rom Mozilla also}
It does have built in security features and spam/junk settings unlike OE. {unless they upgraded it in the last few years}

If interested - more info-
http://www.mozilla.com/en-US/