Things move quickly online, and we’ve beefed up the engine that runs Firefox to make sure you can keep up: Firefox 3.5 is more than twice as fast as Firefox 3, and ten times as fast as Firefox 2.* As a result, Web applications like email, photo sites and your favorite social networks will feel snappier and more responsive.

Link:
http://www.mozilla.com/en-US/firefox/upgrade.html?from=getfirefox

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