I've been with Mozilla for around a year and a half now. I love the hot key options available with it (Ctrl+T) for a new tab, etc. and it's nice being able to customize it a little more than IE 7.0. It sucks some sites still aren't Mozilla friendly, but most handle it well so it's all good.
Jim, he's referring to what web browser you use to access your Internet pages. Computers come with Internet Explorer because of Microsoft owning it, but you can easily download Mozilla Firefox version 2.2.0 (newest) from Mozilla's website and have all your old cookies and favorites transferred to Firefox through the setup process. Personally, I feel it's a superior web browser to IE 7.0, not to mention IE is just now beginning to offer similar features adapted from Firefox.