3 months after Firefox 4.0  has been released, Firefox 5.0 has been released today. I think they are trying to compete with Google Chrome in version numbering.
Mozilla’s shift to a rapid release development cycle delivers cutting edge Firefox features, performance enhancements, security updates and stability improvements to users faster.
The latest version of Firefox includes more than 1,000 improvements and performance enhancements that make it easier to discover and use all of the innovative features in Firefox. This release adds support for more modern Web technologies that make it easier for developers to build amazing Firefox Add-ons, Web applications and websites.
What’s New In Firefox 5.0?
- Added support for CSS animations
- The Do-Not-Track header preference has been moved to increase discoverability
- Tuned HTTP idle connection logic for increased performance
- Improved standards support for HTML5, XHR, MathML, SMIL, and canvas
- Improved spell checking for some locales
- Improved desktop environment integration for Linux users
- WebGL content can no longer load cross-domain textures
- Background tabs have setTimeout and setInterval clamped to 1000ms to improve performance
- Fixed several stability issues
- Fixed several security issues
Looks like they are not done this year, according to Firefox’s Roadmap , Firefox 6.0 and Firefox 7.0 will be released within this year as well.
Direct Download Links To Firefox 5.0
- Official Blog Post: Mozilla Delivers New Version of Firefox – First Web Browser to Support Do Not Track on Multiple Platforms 
- Release Notes: Firefox 5.0