Iterating over result of getElementsByClassName using Array.forEach

I want to iterate over some DOM elements, I’m doing this:

document.getElementsByClassName( "myclass" ).forEach( function(element, index, array) {
  //do stuff
});

but I get an error:

document.getElementsByClassName(“myclass”).forEach is not a function

I am using Firefox 3 so I know that both getElementsByClassName and Array.forEach are present. This works fine:

[2, 5, 9].forEach( function(element, index, array) {
  //do stuff
});

Is the result of getElementsByClassName an Array? If not, what is it?