Firefox's memory leaks are not completely fixed, but Mozilla claims that they have refactored caching methods and timeouts. As of the latest release candidate, I have noticed far fewer crashes.
Under the hood, Firefox 3 reads and understands hypertext code faster than ever. It passes the Acid 2 test with flying colors, and improves greatly on Acid 3.
Firefox 2 vs. Firefox 3 in Acid 2. This is a test to see how compatible a browser is with Web standards.
Firefox will handle more complex fonts and display graphics much better than Firefox 2. You will see improved compatibility for SVG (scalable vector graphics) and better support for offline browsing. HTML5 elements are creeping into the mix, and further support for CSS3 rules has been added.
One of the other great things about Firefox, for developers, is the nightly release schedule for development versions. The need for a new version every day pushes the open-source developer community to really build a solid product. As a producer, I get to see where the action is in browser development, what's working, and what's stalling out.
Speaking of those updates, I'm looking forward to Firefox 3.1, code-named Shiretoko, which will be rolled out by the end of 2008. (You can download alpha 1 today.) This version of the browser will have more support for CSS3 and more native integration XHR (XMLHttpRequest). More support for CSS3 means that we can support newer design elements that weren't possible before; better XHR means that our Ajax applications will run more smoothly. Users may or may not notice these simple fixes at first, but the result should be faster and more reliable Web apps.