Honestly? I switched from IE to Firefox because I couldn't seem to avoid getting malware infections. Even after changing antivirus programs several times and TRYING my best to be careful, IE would continuously get hijacked and infect me with malware, which led to constant repairs and more often than not reformatting my computer.
Firefox along with Adblock and Noscript helped to put a stop to this problem, and I've been malware free ever since. Noscript tends to get very annoying at times, but it's a fair trade for the feeling of security.
My problem was I really loved the browser interface and functions of IE, so I was hesitant to make the switch. I don't use tabs for browsing and I didn't like most of the features in Firefox that apparantly couldn't be de-activated. Thankfully a computer-savy friend helped me to reconfigure Firefox to match that which I was more comfortable with.
At its core, I just really wanted a more secure browser, since IE has the defense of wet tissue paper. I'm glad I made the switch just in time, since shortly afterwards the newest version of IE came out and it is just HORRIBLE.