I was looking at Mozilla Developer Documentation on Javascript. Is it Mozilla's interpretation of the ECMAScript standard or is it documenting how they have implemented Javascript in Firefox?
Basically, I want to know whether their documentation is valid across all browsers or just Firefox.
It's both, basically. From JavaScript/Reference/About:
The JavaScript Reference serves as a repository of facts about the JavaScript language. The entire language is described here in detail. […]
Recent versions of Mozilla-based browsers support newer versions of JavaScript. […]
Browsers that do not support at least JavaScript 1.5 are very rare today, since JavaScript 1.5 was introduced back in 1999. If you're interested in historic information, please refer to the Wikipedia article on ECMAScript.
JavaScript documentation of core language features (pure ECMAScript, for the most part) [consists of the Guide and the Reference].
It is definitely a reference about the Mozilla implementation(s) of JavaScript, which today covers all of EcmaScript 5.1 features. They're well documented, also containing information about bugs in older Mozilla implementations and in relevant other engines. Each article also lists cross-browser support in a table, though these are sometimes not correct and/or exhaustive.
The reference also includes documentation of proprietary Mozilla features and of their draft implementations for upcoming standards. These are properly marked as such, usually with the non-standard-tag.
Also, don't forget that it's a wiki!
as Alex K has said in his comment It documents Geko, the mozilla implementation of JS which also includes all the non-standard functionality they have included
It is also an excellent reference for standard js api calls, but should always be used with a slight caveat of, it is the Geko implementation so may not behave as described, but generally it does in my PERSONAL experience
