http://jjant.com

Here describes how to use XulRunner throw XPCOM for editing html/txt nodes

First of all we should setup editor for window or put mozilla into design mode, it's should be done when document will be loaded complitely:
• Putting xulrunner browser into design mode using nsIDOMNSHTMLDocument
  

  nsIWebBrowser webBrowser = (nsIWebBrowser)getBrowser().getWebBrowser();
  webBrowser.getContentDOMWindow();
  nsIDOMWindow iDOMWindow = webBrowser.getContentDOMWindow();
  nsIDOMDocument iDOMDocument = iDOMWindow.getDocument();
  nsIDOMNSHTMLDocument iDOMNSHTMLDocument = (nsIDOMNSHTMLDocument)iDOMDocument.queryInterface(nsIDOMNSHTMLDocument.NS_IDOMNSHTMLDOCUMENT_IID);
  iDOMNSHTMLDocument.setDesignMode("on");

• Putting xulrunner browser into editiong mode using nsIEditingSession:
  

  //here we creating editing session using component manager
  nsIEditingSession editingSession = (nsIEditingSession)getComponentManager().
  createInstanceByContractID("@mozilla.org/editor/editingsession;1",null,nsIEditingSession.NS_IEDITINGSESSION_IID);
  //here we make window editable using editing session
  editingSession.makeWindowEditable(iDOMWindow, "html", true);
  //setups editor for window
  editingSession.setupEditorOnWindow(iDOMWindow);
  //getting an instance of editor for window
  nsIEditor editor = editingSession.getEditorForWindow(iDOMWindow);
  //setups editor action listeners
  editor.addEditActionListener(getEditActionListener());
  //setup flags using which we can disable or anable some editor actions
  editor.setFlags(nsIPlaintextEditor.eEditorReadonlyMask);
  //code with whic we are getting selection controller
  selectionController = editor.getSelectionController();
  //with this code we are getting object resizer and we can use it's for controlling resizing
  nsIHTMLObjectResizer htmlObjectResizer = (nsIHTMLObjectResizer) editor.queryInterface(nsIHTMLObjectResizer.NS_IHTMLOBJECTRESIZER_IID);
  //we disable abject resizers
  htmlObjectResizer.hideResizers();
  htmlObjectResizer.setObjectResizingEnabled(false);
  //here we getting position editor and disable it's too
  nsIHTMLAbsPosEditor htmlAbsPosEditor = (nsIHTMLAbsPosEditor) editor.queryInterface(nsIHTMLAbsPosEditor.NS_IHTMLABSPOSEDITOR_IID);
  htmlAbsPosEditor.setAbsolutePositioningEnabled(false);
  //here we getting inline table editor and disable it's too
  nsIHTMLInlineTableEditor inlineTableEditor = (nsIHTMLInlineTableEditor) editor.queryInterface(nsIHTMLInlineTableEditor.NS_IHTMLINLINETABLEEDITOR_IID);
  inlineTableEditor.setInlineTableEditingEnabled(false);

Description of nsIEditingSession
Very Short description of interfaces that are was using in it's example