Few years ago, I found Firefox, coming from AOL 9.
Never looked back.
Personally, I find that Firefox, compared to IE6 or IE7, is faster to start, to load web pages, and even to download files.
It's also more customizable for the advanced user or the beginner, and arguably more secure.
The main things I like are tabbed browsing, which I vastly prefer to IE7's implementation, and the add-ons and themes.
If you're not sure, just give it a try. It's free and it won't hurt to just try it out for a few days, then you can see which you like better.