FF13 comes out with experimental support for ECMAScript 6 Map and Set objects.

As maps and sets can be easily implemented without this support, they have been introduced as experimental support

It's important to note :

An Object has a prototype, so there are default keys in the map. However, this can be bypassed using map = Object.create(null). The keys of an Object are Strings, where they can be any value for a Map. You can get the size of a Map easily while you have to manually keep track of size for an Object.

So far Maps were implemented as normal objects  {} and sets were implemented with a simple filter like [].sort.filter( function(v,i,o){return v!==o[i-1];}.

But now FireFox 13 being the first browser to support these two APIs ( Seems more Java like though)

Maps in Javascript with FF13 :

Map operations : map.get(key); map.set(key, value); map.has(key); map.delete(key); map.size();

>>> map = new Map();
Map {}
 
>>> map.set("GNU","Fun!");
 
>>> map.size
size()
 
>>> map.size()
1
 
>>> map.get("GNU")
"Fun!"

Sets in Javascript with FF13

Set operations : set.add(value);set.delete(value);set.has(value);set.size();

SAD we still can't iterate over it! :(

>>> var set = new Set();  
undefined
 
>>> set.has(420)
false
 
>>> set.add(420)
undefined
 
>>> set.has(420)
true
 
>>> set.delete(420)
true
 
>>> set.has(420)
false

Important to note!

This is just a prototype implementation of the Set and Map API's 
it is (and will) subject to change anytime.

Let's wait and watch how many will vote for these APIs :)