The Mozilla corporation has moved to a rapid release development cycle, similar to Google and Chrome, which means that now, after only three months, Firefox 5 is available for download.
Personally, I found Firefox 4 to be quite sluggish all round, but the same cannot be said for Firefox 5. It seems to be lightning fast.
The main changes with Firefox 5 include:
Better support for web standards
Bug fixes
Support for CSS animations
Better visibility for the Do-Not-Track header preference
Improved canvas and JavaScript support
Better standards support for canvas, HTML5, XHR, MathML and SMIL
Better tuned HTTP idle connection logic
General performance enhancements