Great! It's fun to see the browser wars in full swing again. Competition makes better browsers for everyone.
Firefox 6 has WebSockets support as well - a very interesting one for possibly developing multiplayer games...
Websockets, WebM, data- class of attributes. This could be used to get parameters and infos from the webpage.
Also Firefox 6 for developpers.
Updated my FF, installed Firebug 1.8.1 and I was ready to rock.
Also Ash: they seem to have done something with the audio support.
I must admit I didn't quite get that part.
2nd EDIT: Interesting page.
SVG, APIs, CSS formating, CSS unit scaling, WebGL.