- 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
- Fixed several stability issues
- Fixed several security issues
See the release notes for more details. Firefox 5.0 replaces Firefox 4.0.1 which as now reached end of life support. Next release will be in about 8-weeks with Firefox 6 on August 16th.