SlimerJS is a scriptable browser. It allows you to manipulate a web page with a Javascript script: opening a webpage, clicking on links, modifying the content... It is useful to do functional tests, page automaton, network monitoring, screen capture etc.
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. See current release notes in docs/release-notes.rst, and read the compatibility table to know the implementation progress.
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 1.1beta 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 XULRunner (or optionaly with any recent version of 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 stable release 1.0 is to have a full implementation of the API of PhantomJS 1.9.1, with the support of Coffee scripts and Ghost Driver.
After this release, the goal will be to hack XulRunner to run headless windows.
Read the faq into the website/faq.html file or on the website.