The Preview Release of Firefox version 1 is available now from Mozilla.
It is now the main download file on the Firefox homepage.

This version offers 4 major new features:

• Livemarks - RSS integration in the browser
• FastFind bar - Small toolbar allows you various options for finding text on a page
• New Plugin installer
• Warning toolbar for pop-ups and software installs

As well as some more minor updates:

• Bookmark manager improvements
• Improved password encryption
• Improved compatibility for IE users
• Firefox can also now be configured as the default browser on GNOME.

Read the improvements in more detail here, here and here.

Unfortunately, I've found a lot of my favourite extensions will not work with this new version, though eventually most should be upgraded by the various authors.

More info:
Firefox Preview Release Available, Spreadfirefox.com in Beta
blakeross.com - it's here.