How can I access data elements from multiple browser windows?

I have five separate browser windows all open corresponding to one website. For simplicity's sake, it's kind of like a game with multiple windows of the game open. Each of these windows has an html element that I want to add together and just have a running total of without having to do it manually. Is there any way I could easily access data from all of these different windows simultaneously? Would something like a chrome/firefox extension be the way to go?

Thanks!