I'm pretty sure Outlook works with FF. I use Thunderbird (you can import all your Outlook stuff to Thunderbird...) now, but I used to use Outlook and am fairly certian I used FF and OL together.
That whole "hackers follow the masses" thinking is correct, to an extent. FireFox is open source, that means ANYONE can freely download the source of the program, including hackers. But, because it is open source, there are very brilliant minds who write the program and make it so that hackers can't breach the security of the program. Is it impossible to hack FF? No, but it is much less likely. Also, since it is open, the program reacts very quickly to any breaches and flaws. In fact, you can download a new version of FF every nite. It isn't recommended as much of the stuff involved in "nightly releases" is in alpha testing at best.
Anyway, I highly recommend FF. I've been using it for more then a year now and haven't looked back. The features are just incomparable, the ease of use and flexibility is great as well. It's a free program that downloads quickly, try it (you don't have to get rid of IE) and you'll never turn back. You don't have to change anything to use it.