This means pages load faster, interactions with websites are snappier and the Web is just a lot more fun. For developers, this means you can build richer high-performance Web applications and explore the world of 3D graphics, inherent to the Web.
Also, the Add-ons APIs for Firefox 4 are now stable, so if you are an add-on developer, now is the time to update your Firefox 3.6-compatible Add-ons to support Firefox 4.
Firefox 4.0 Beta Performance Graph
Boosting Graphics Performance
This update to Firefox 4 Beta incorporates hardware-accelerated graphics into the final rendering of a website — referred to as “compositing.” This enables websites to load and respond faster to rich and interactive content like Web games, apps or photos. On Windows (including Windows XP), hardware acceleration is done using DirectX technology; on Mac OS X, it’s done using OpenGL.