I prefer to use Firefox. Here are the pros and cons of each so that you can make a decision based on your preference.
Firefox:
The good: Firefox 2 adds built-in antiphishing protection, search engine suggestions, session restore, inline spell-checking, and Live Titles; the browser is available for Windows, Mac, and Linux; localized versions available in many different languages.
The bad: The Firefox 2 uninstall leaves behind a mess; some 1.5 version add-ons will break in 2.0; there are no thumbnail previews of open tabs; the browser doesn't yet pass the Web Standards Project Acid2 test.
The bottom line: Mozilla Firefox 2 is a winner, beating Microsoft Internet Explorer 7 on security, features, and overall cool factor.
Internet Explorer:
The good: IE 7 includes built-in tabbed browsing; antiphishing technology; an RSS reader; and a redesigned Favorites Center.
The bad: IE 7 is limited to Windows XP SP2 users or up only; installation requires reboot; reuses old IE 6 code and doesn't yet comply with current Web standards; doesn't match all the features found in Firefox or Opera; carries a Microsoft legacy of not patching its IE flaws quickly enough.
The bottom line: IE 7 was Microsoft's one chance to leapfrog ahead of the competition, but the company has only barely caught sight of the current front-runners. For more features and greater security, switch to Mozilla Firefox.