jQuery .serializeObject is not a function - only in Firefox

I'm using jQuery, and specifically this function

$("#postStatus").serializeObject();

It works absolutely fine in Chrome and Safari, but when I do it in Firefox it doesn't work. I used Firebug to see what error it was giving, and i'm getting this

$("#postStatus").serializeObject is not a function

Why doesn't this function work in Firefox?

UPDATE...

Oh yes, I completely forgot that it's not a core function. I remember that I searched a way to serialize a form and found this solution;

$.fn.serializeObject = function()
{
   var o = {};
   var a = this.serializeArray();
   $.each(a, function() {
       if (o[this.name]) {
           if (!o[this.name].push) {
               o[this.name] = [o[this.name]];
           }
           o[this.name].push(this.value || '');
       } else {
           o[this.name] = this.value || '';
       }
   });
   return o;
};

I've managed to fix this issue by placing the function above at the top of the JS file. Thanks for your help guys.