Firefox 3.5
HTML5
Some really practical features of HTML5 are the audio and video elements that can be embedded without Flash, Quicktime or Quicksilver.
If you’re a web developer take a look at this breath-taking video to see the potential: HTML5, Canvas, Video demo on Youtube
Downloadable fonts
The CSS @font-face rule will allow fonts to be displayed in web browsers above and beyond the web-safe fonts currently available. And without unwieldy technologies like sIFR.
TraceMonkey
If you run heavy Javascript in your browser (e.g. for applications like Gmail) then you’ll welcome TraceMonkey, the new Javascript engine.
It speeds up response times drastically being close to Google Chrome, over twice as fast as Firefox 3.1 and around four times faster than IE8.