A oneline helper I put in all of my apps. Looks for HTTP headers that signal you're being called via an XmlHttpRequest. I use it for instant degradable AJAX by rendering a template if it's as usual but just rendering a partial if they just want that part!

  def ajax_request?
    request.env['HTTP_X_REQUESTED_WITH'] == 'XMLHttpRequest' || params[:ajax]  # params[:ajax] for safe-keeping, FIXME if it works w/ all browsers
  end

Firefox doesn't do a lot of simultaneous AJAX (or any kind of HTTP) requests. IE caps it at 2, too. This will allow you to test your XHR overload scripts, or just load pages faster in general.

1. Go to "about:config"
2. Find and edit the following
* network.http.pipelining=false
* network.http.pipelining.maxrequests=4
* network.http.proxy.pipeline=false
3. Make the false's true; I set my maxrequests at 20