SlimerJS A scriptable browser for Web developers
A Versatile Scriptable Browser – SlimerJS SlimerJS is a scriptable browser for web developers allowing manipulation of web pages with external Java scripts. It is useful to carry out functional tests, network monitoring, page automation, screen capture and more. In many ways it shares the common aims with PhantomJS, but runs Gecko, the core of Mozilla Firefox instead of Webkit. SlimerJS provides the same API of PhantomJS and the current version of SlimerJS is highly compatible with the later. However, because PhantomJS is built on top of Webkit, it is not conducive for developers to carry out or run scripts with any other web engines like Gecko. SlimerJS is not headless but with the use of xvfb under Linux or MacOS, headlessness can be emulated. SlimerJS is a XUL/JS application that is launched with XULRunner or with any of the Firefox versions in vogue. SlimerJS allows execution of all the scripts developed for PhantomJS and that is also one of its chief purposes. Tools like CasperJS can be executed with SlimerJS as well, adding on to its capabilities and versatility.
The Feature List SlimerJS 
SlimerJS most importantly allows execution of JavaScript outside a classical web browser
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Allows opening a webpage
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Clicking on web links
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Modifying content
Modules can be used within the JavaScript to integrate various APIs in SlimerJS to o
Read and write files
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Load a web page and inspect the content of a web page
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Emulate user behavior on a web page to test or examine the results
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Network monitoring by listening
Modules of the Mozilla Add-on SDK are available which can be imported very easily for fruitful use
The web page rendering in SlimerJS is identical to the rendering in Firefox
Loading of Flash content is possible if the Flash plugin is installed
SlimerJS allows execution of Common JS modules including CoffeeScript modules
All of these features are executed on top of the core of Firefox and it goes without saying that SlimerJS gives an edge over and above the core Firefox advantages.