To be perfectly honest I cant see any difference with the icons.
But firefox 3 is a major step forward for Mozilla. Firefox runs much more smoothly now and uses nowhere near as much memory as previous versions. And the new location/address bar is amazing, I actually have problems using the old firefox and IE now because it is so easy to jump to different websites.
The only reasons why I haven't upgraded all of my computers to firefox 3 are because only a handful of add-ons and themes work, and flash videos (not animations) make firefox crash, but I think this is because of my laptop's low specs.
Otherwise it is definitely worth upgrading, despite the fact that it is still in beta.
Hope this helps you