Denise,
Since the idea behind these Mozilla products usually is to move away from Microsoft products, there currently isn't a toolbar add-on for Outlook express in FireFox. There is at least one extension that will let you send things to Outlook Express, but not a button as you describe.
There is an extension for anyone who has a gmail account that I love. It puts gmail at the bottom of the page, and tells you when you have mail, and allows you to open gmail with one click. (If you don't have a gmail account and want one, let me know, I will send you an invite.)
You might not surf much now, but Stumble will have you addicted to "surfing" stumble-style in no time!!!