This post has been published more than 10 years ago, it may
be obsolete by now.
I’m happy to annouce the immediate availability of CasperJS 1.1-beta1, featuring support for SlimerJS, which basically ports the PhantomJS API onto the Gecko platform.
Yes, that means as of 1.1-beta1 you can run most of your existing CasperJS scripts against a headless Firefox (using a virtual framebuffer for now), thanks to the huge amount of effort provided by Laurent Jouanneau, a long-time XUL/Gecko contributor.
This is great news for all Web developers wanting to avoid contributing to the establishment of a monoculture.
1.1-beta1 brings a whole lot more other features, you may want to read the full CHANGELOG.