This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Taking a look at the discussion you mentioned, is the same issue, but i don't really think to the problem is firefox, i'm using chrome and seems like if you sent the exact same request, the container reuse the request and process it one after the other or something like that.
So anyways in order to actually see the same servlet running on diferent threads at the same time i changed the following:
It isn't the container which does that, it's the client (in this case the browser). You can enable "Developer Tools" for Chrome or Firebug for Firefox and verify whether or not the requests are sent simultaneously or sequentially by the browser.