Very often when I find Firefox consuming excessive number of my cycles...and I go looking there is a site that has a runaway Javascript working.
Or a Flash app...which is likely using Javascript....as I understand it....
Firefox is reworking and soon to release 3.5 so... each tab has its own process so you can kill the offending tab without taking all of firefox down...
So perhaps this V8 addresses the Javascript problems...also...
Watching the second video...says they are using Webkit...not sure what that is...