InfoTech: Firefox upgrade has record downloads

Mozilla says (edited)

Firefox 3.5 includes the powerful new TraceMonkey JavaScript engine. It's twice as fast as Firefox 3.0 - and on complex sites more than ten times faster than Firefox 2.0.

OGG open format video and audio is built into the browser: no need for plugins.

Location-aware browsing. Tell your browser where you are, and it will deliver geo-location relevant content.

Privacy. "While using the new Private Browsing mode in Firefox 3.5, nothing you encounter on the Web will be stored from that moment on during your browsing session. Unique to Firefox 3.5, the new Forget this Site feature can remove every trace of a site from your browser. If you want to remove all private data or activity from the past few hours, Clear Recent History, another Firefox-only feature, gives you full control over what stays and what goes."

http://www.mozilla.com/en-US/firefox/all.html