Enumerating tabbrowser tabs

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• To go through all open tabs in a browser, you first need to get a reference to browser's window. If your code is executed from a browser.xul (in Firefox) overlay (for example it is a toolbar button or menu click handler), you can access current window via window pre-defined variable. If your code is executed from its own window (for example, Settings dialog), you can use nsIWindowMediator to get it.
• Then you need to get the <tabbrowser> element. You can get it with win.gBrowser, where win is the browser's window from the previous step. You can use just gBrowser instead of window.gBrowser, if running from browser.xul overlay.
• Now use gBrowser.mPanelContainer.childNodes.length to get the number of open tabs. Then use gBrowser.getBrowserAtIndex() to get a <browser> element. For example:

  var l = gBrowser.mPanelContainer.childNodes.length;
  for(var i = 0; i < l; i++) {
    var b = gBrowser.getBrowserAtIndex(i);
    try {
      dump(b.currentURI.spec); // dump URLs of all open tabs to console
    } catch(e) {}
  }

To learn what methods are available for <browser> and <tabbrowser> elements, use DOM Inspector or look in browser.xml and tabbrowser.xml for corresponding XBL bindings.