It is 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 (but not on MacOS).
SlimerJS provides the same API of PhantomJS. The current version of SlimerJS is highly compatible with PhantomJS 1.9. See current release notes in docs/release-notes-*.rst, and read the compatibility table to know the implementation level.
The main goal of SlimerJS is to allow to execute all scripts developed for PhantomJS. So you could use tools like CasperJS. In fact, CasperJs 1.1 and higher can be executed with SlimerJS!
SlimerJS is not only a PhantomJS clone, it contains also additional features.
Technically, SlimerJS is a XUL/JS application that is launched with Firefox.
Follow us on twitter: @slimerjs
Ask your questions on the dedicated mailing list.
Or discuss with us on IRC: channel #slimerjs on irc.mozilla.org.
See documentation into the docs/ directory into the source code, or read it on the web site
The goal for a first major stable release 1.0 is to have a full implementation of the API of PhantomJS 1.9.x and 2.1, with the support of Coffee scripts and Ghost Driver.
Read the faq into the website/faq.html file or on the website.