FireFox is a stand alone browser that has no connection to MS IE. I think it is better for a number of reasons. it is extensible(you can add different functionality if you want through other small programs people make for it), skinnable(people can also make custom skins), it has built-in pop-up blocker(which IE will get with SP2 for XP) and has tabbed browsing. Tabbed browsing is where there youcan open a new page in tabs instead of a whole new window.
One thing to remember though. it is only 0.9 so it isn't quite done. It is still labled as a beta. I have used it since 0.5 or so with no major problems. The only bad thing that I can see with 0.9 is the new default skin. it looks as if it isn't finished and is a bit clunky. The skin is clunky, not the interface which is pretty clean and straight forward.
You can still use Outlook Express just fine as FireFox doesn't come with an email client. If you want to try the related email client, Mozilla Thunderbird 0.7 is also out.
As for security measures, I would imagine that it is more secure as it isn't attached to the OS like IE is. As to how much more secure, I can't really say. I'm not a software specialist.
Hope this helps.