From the Mozilla Blog:
Today we’re releasing the first beta-quality version of Firefox 4, which starts us down the path to a final release of Firefox 4. We’re handling this beta differently than we’ve done other releases. In previous betas we’ve made milestone-like releases. For this beta we’ll be making more frequent updates during the beta program. So if you download the beta build and run it you’ll likely get updates every two to three weeks, instead of a couple of months apart. We believe that this will give us the ability to reply to people’s feedback quickly and get fixes and changes tested earlier. This, in turn, will mean we’ll be able to release a much higher quality browser as a result.
Performance
Firefox 4 contains a large number of performance improvements over Firefox 3.6. As a web developer you’re likely to notice big improvements in overall performance.
DOM and Style Performance – We’ve made huge improvements in our DOM and style resolution engine, meaning that pages that have complex CSS rules and selector matching will generally work faster and better. (On some tests in the Zimbra performance test suite we’ve seen a solid 2x improvement.)
Reduced IO in the page load path – One big area where we’ve made huge improvements in Firefox 4 vs. 3.6 is the removal of tons of I/O from the main UI thread. This means making sure that when we do history look-ups for coloring links based on browser history, that those look-ups are done off the main thread, to making sure that we’re not synchronously writing data to the HTTP cache on the main thread. This alone has improved the overall feel of the browser more than anything else.
JavaScript – The JavaScript engine is much faster as well, although beta 1 does not include the new JägerMonkey work. That work is well underway and will be landing through the beta process, and is already showing positive results.
The full article can be found here