Endless Paradigm

Have not tried it yet, but for people who use Firefox, this may be of interest.
Beta has been out for a while, so you may already know of the new features.
Summary of main features from Mozilla blog

Quote:Fastest Firefox Ever

• Performance: Firefox is up to six times faster than the previous release. With improved start-up and page load times, speedy Web app performance and hardware accelerated graphics, Firefox is optimized for rich, interactive websites.

Streamlined Interface

• App Tabs: give a permanent home to frequently visited sites like Web mail, Twitter, Pandora, Flickr.
  
• Switch to Tab: easily find and switch to any open tab from your Awesome Bar without opening duplicate tabs.
  
• Panorama: drag and drop tabs into manageable groups to save time while navigating many open tabs.

Private and Secure Synchronization

• Firefox Sync: access your Awesome Bar history, bookmarks, open tabs, passwords and form data across multiple computers and mobile devices.

Most Customizable

• New Add-ons Manager: easily discover and install more than 200,000 add-ons to customize the features, functionality and look of Firefox.

Private and Secure

• Do Not Track: Firefox is leading the Web towards a universal standard Do Not Track feature that allows users to opt-out of tracking used for behavioral advertising.
  
• Firefox puts privacy first, fixing flaws in some Web standards to prevent others from accessing your browser history.
  
• HTTP Strict Transport Security (HSTS): automatically establishes secure connections to stop “man in the middle” attacks and keep sensitive data safe from interception during the log-in process.
  
• Content Security Policy (CSP): prevent cross-scripting attacks by allowing sites to explicitly tell the browser which content is legitimate.

Cutting Edge Tools for Web Developers:

• The JavaScript engine incorporates the new JägerMonkey JIT compiler, along with enhancements to the existing TraceMonkey JIT and SpiderMonkey’s interpreter for faster page-load speed and better performance of Web apps and games.
  
• HTML5 support in Firefox includes hardware accelerated, high-definition video (WebM), 3D graphics, offline data storage, professional typography, touchscreen interfaces and the Mozilla Audio API to help create visual experiences for sound and more.
  
• Firefox 4 also improves existing tools like CSS, Canvas and SVG to enable developers to make exciting Web pages.
  
• Firefox provides uninterrupted browsing when there is a crash in the Adobe Flash, Apple QuickTime or Microsoft Silverlight plugins. If one of these plugins crashes or freezes, it won’t affect the rest of Firefox. Simply reload the page to restart the plugin.

Download
Portable version

No official Windows x64 version as of yet it seems.  I guess they may have thought that it ate too much RAM?

Firefox 4 is running great on OS X. The final version came out 2 days ago for me via self update. It uses much less RAM about 300-400MB, down from 600-700MB. The only bug I've seen is when running a flash movie in full screen mode it sometimes goes back to a window.

xero1 Wrote: [ -> ]It uses much less RAM about 300-400MB, down from 600-700MB.

O.o

How many tabs do you have open?  How many with heavy flash content?

My Firefox doesn't use even remotely close to those numbers...

I've just installed it and must say i can see quite a difference in performance already

Assassinator Wrote: [ -> ]

xero1 Wrote: [ -> ]It uses much less RAM about 300-400MB, down from 600-700MB.

O.o

How many tabs do you have open?  How many with heavy flash content?

My Firefox doesn't use even remotely close to those numbers...

4-8 tabs, with little flash.

More or less?

With 4 tabs open.

my firefox only uses about 100MB RAM

definitely a lot snappier

I'm using it right now, runs pretty good.

This one is much faster than the beta's

Using it now, I like it.

Opera with 5 tabs open