This is an intermediate type release.
The upgrade to Gecko is nice, but the big change, dropping Spider Monkey and using Tamarin won't happen until Firefox 4.
When that happens, Firefox will be ECMAScript 4/JavaScript 2 compliant and the Tamarin interpreter is blazing ******* fast. Right now you are starting to see RIAs built from JavaScript frameworks instead of Flash. That's all in its infancy. Firefox 4 will allow JavaScript-based sites to fully mature.