firefoxos marketplace count total donwloads

// first select start and end dates
var sum = 0;
var a = document.querySelectorAll('.bar-wrapper.single > ul > li > span > em');
var l = a.length;
for(var i=0;i<l;i++)
    sum = sum + Math.round(a[i].childNodes[0].nodeValue);
    
window.alert(sum);