Part of Mozilla`s claim is that the speed is much faster, and I`d agree with that.
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What’s New in Firefox 3.5
Firefox 3.5 is based on the Gecko 1.9.1 rendering platform, which has been under development for the past year. Firefox 3.5 offers many changes over the previous version, supporting new web technologies, improving performance and ease of use.
Super Speed
Fasten your seatbelt: Firefox 3.5 includes the TraceMonkey JavaScript engine, which gives the browser dramatically better performance than ever before. Firefox has always been fast, but this is the fastest Firefox ever (more than twice as fast as Firefox 3, and ten times as fast as Firefox 2), meaning Web applications like email, photo sites, online word processors and more will feel snappier and more responsive.