This is currently where Mozilla Firefox is going, and they've released a public preview of where they believe the web will be when they release it. The main drawbacks of installing this are definitely the lack of plugin/extension support and coming across possible bugs.
You can install this version in parallel to other versions of Firefox (I'm running 4.x). But to be honest, I hardly see any real new features in this release of Firefox.
The current release plan for Firefox versions is Aurora, then Beta and Final.
Mozilla promises to improve on performance with future Firefox versions and this is the starting point.
Update: Firefox 5 is now available.
From Mozilla:Experience the newest innovations in an unstable environment thatís not for the faint of heart.Firefox Aurora Beta 7.02a2
is licensed as Freeware
for the Windows operating system / platform without restrictions. Firefox Aurora Beta is available to all software users as a free download (Freeware).
Firefox Aurora Beta Virus and Malware Tests:
For security reasons, you should also check out the Firefox Aurora Beta virus and malware test
that has been uploaded; additional Firefox Aurora Beta screenshots may be available, too.