OK, I'll give it a shot.
First, the one that you are bound to have heard on a web design forum - it's a standards complient browser. Pages that look good in Firefox are guaranteed to work in other standards complient browsers - Opera etc. Sites that looks good in IE are not guaranteed to work in anything but IE - true IE does take a market share but what about the 15% (and growing) that don't use IE. Do you just forget about them? Not very nice.
It's more secure as mentioned.
Live bookmarks are fantastic. I have the BBC News & Sport headline available to me. I click on a button saying "Latest Headlines" and a menu pops down with all the headlines. Nice for keeping up to date with the latest news. Also adding these bookmarks is very, very easy. It's a case of going to an appropriate page (e.g.
http://news.bbc.co.uk/sport) and hitting a button.
Extensions are fantastic - can you control Winamp from IE? Nope. Can you see how many new GMails you have from IE? Nope. Can you see the weather without moving? Nope. The possibilities are truly endless.
Tabbed browsing. I hate having 101 things open on my taskbar. Now I can have just window containing 5 web pages and just CTRL+TAB through them.
The built in web search is top notch. It can be changed to use a lot (and I mean LOTS) of different search engines. Saves the need for a load off different IE toolbars. I can search Google, IMDB, GameFaqs, Wikipedia and more all from the same text box.
Built in download manager
I'm yet to see a bit of spyware affect my computer with Firefox's help.
Skins if you want to change the look of your browser.
It's open source. Bugs are found more quick and fixed faster. Also you know that the code isn't doing something sneaky as you could simply check yourself. IE could be doing all kinds of stuff and we would never know.
That'll do for now. Rick, you said that spyware tools and anti-virus tools kept you safe but they do say that prevention is better than a cure.