Although the Firefox Version 2.0 Beta is not actually released until Tuesday 11th, you can download the feature complete Windows public release candidate 1 build right now from the Mozilla FTP server.
Linux and Mac versions are also available, early reports suggesting that it works just fine with Intel based Macs at last.
Apart from the integrated spell checker and anti-phishing filter, Firefox 2.0 adds horizontal scrolling for tabs and a revised options dialog. Most of the changes, though, are beneath the surface.
Do not expect your existing extensions to work. Do expect your existing themes to be overwritten. Unless, that is, you download the portable version instead as this keeps everything within the Portable Firefox folder which can be uninstalled simply by deleting it. Your existing profile remains separate and untouched. This seems the sensible option, given that much of the value of Firefox is invested in the extensions it supports.