It is in fact a tool like PhantomJs, except that it runs Gecko instead of Webkit, and it is not (yet) natively headless. However, it can be headless with the use of xvfb under Linux or MacOS. SlimerJS provides almost the same API of PhantomJS. The current version of SlimerJS is highly compatible with PhantomJS even if it is not 100% compatible yet. SlimerJS is not only a PhantomJS clone, it contains also additional features.
License: MPL 2.0 License