The new version, Firefox 5, has incremental changes from the last one, but the most notable is the branding– succeeding Firefox releases will be dropping the version number from its branding. (But the version number can still be found in the “About Firefox” section.)
Why so soon? This new release is part of Mozilla’s new rapid release cycle (partially inspired by Google’s approach to churning out new versions of Google Chrome every so often). With this new rapid release cycle, we may see Firefox 7 by the end of the year.