Firefox is way superior to IE. But some specific sites are optimized for IE, and some will run only on IE. So you do want to keep both. Use Firefox first and if it doesn't work right, cut and paste the page into IE.

One great thing about Firefox is all the available add-ons. Just click and add. The one I use most is the weather one: it keeps info in the little bar at the bottom of the screen at all times with current temp, today's high, tomorrow's high ... and a little red exclamation mark comes up anytime bad weather is on the way. All clickable for more info of course.