And while people wait, you can already do a 'poor mans' headless Firefox thanks to SlimerJS and xvfb.

Phantomjs is less resource heavy if you're constantly spooling up and down lots of instances but I prefer SlimerJS w/ Firefox since it lets you keep up to date with a modern version of Firefox (rather than relying on sporadic QTWebkit updates from Phantomjs).

If you're using Casperjs, SlimerJS is virtually a drop in replacement for Phantomjs (though I worry about how long/well Casperjs will continue to be maintained).