How do you make a post request into a new browser tab using JavaScript / XUL?

var windowManager = Components.classes["@mozilla.org/appshell/window-mediator;1"]
    .getService(Components.interface
s.nsIWindowMediator);
var browserWindow = windowManager.getMostRecentWindow("navigator:browser");
var browser = browserWindow.getBrowser();
if(browser.mCurrentBrowser.currentURI.spec == "about:blank")
    browserWindow.loadURI(url, null, postData, false);
else
    browser.loadOneTab(url, null, null, postData, false, false);

function openAndReuseOneTabPerURL(url) {
  var wm = Components.classes["@mozilla.org/appshell/window-mediator;1"]
                     .getService(Components.interfaces.nsIWindowMediator);
  var browserEnumerator = wm.getEnumerator("navigator:browser");

  // Check each browser instance for our URL
  var found = false;
  while (!found && browserEnumerator.hasMoreElements()) {
    var browserInstance = browserEnumerator.getNext().getBrowser();

    // Check each tab of this browser instance
    var numTabs = browserInstance.tabContainer.childNodes.length;
    for(var index=0; index<numTabs; index++) {
      var currentBrowser = browserInstance.getBrowserAtIndex(index);
      if ("about:blank" == currentBrowser.currentURI.spec) {

        // The URL is already opened. Select this tab.
        browserInstance.selectedTab = browserInstance.tabContainer.childNodes[index];

        // Focus *this* browser
        browserInstance.focus();
        found = true;
        break;
      }
    }
  }

  // Our URL isn't open. Open it now.
  if (!found) {
    var recentWindow = wm.getMostRecentWindow("navigator:browser");
    if (recentWindow) {
      // Use an existing browser window
      recentWindow.delayedOpenTab(url, null, null, null, null);
    }
    else {
      // No browser windows are open, so open a new one.
      window.open(url);
    }
  }
}