Mozilla Firefox 3.0

No longer a fledgling upstart, Firefox 3 is full-featured, lightning fast, and an able foe for both the big-dog competitor Internet Explorer and competitors nipping at its heels. Firefox's killer add-ons remain strong, and the latest update makes version 3.5 about two times faster than version 3. However, competition is strong and it can no longer be said that Firefox is the fastest browser available.
Firefox Performance: Fast — Faster — Fastest. Results of a SunSpider test on a Windows XP machine.

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