_ff-addon-snippet-HighlightTextInDocument - This takes from http://mxr.mozilla.org/mozilla-release/source/toolkit/modules/Finder.jsm#225 to try to highlight text in document.

_ff-addon-snippet-HighlightTextInDocument.js

  function _getEditableNode(aNode) {
  	while (aNode) {
  		if (aNode instanceof Ci.nsIDOMNSEditableElement)
  			return aNode.editor ? aNode : null;

  		aNode = aNode.parentNode;
  	}
  	return null;
  }

  function _highlightRange(aRange, aController) {
  	let node = aRange.startContainer;
  	let controller = aController;

  	let editableNode = this._getEditableNode(node);
  	if (editableNode)
  		controller = editableNode.editor.selectionController;

  	let findSelection = controller.getSelection(Ci.nsISelectionController.SELECTION_FIND);
  	findSelection.addRange(aRange);

  	if (editableNode) {
  		// Highlighting added, so cache this editor, and hook up listeners
  		// to ensure we deal properly with edits within the highlighting
  		if (!this._editors) {
  			this._editors = [];
  			this._stateListeners = [];
  		}

  		let existingIndex = this._editors.indexOf(editableNode.editor);
  		if (existingIndex == -1) {
  			let x = this._editors.length;
  			this._editors[x] = editableNode.editor;
  			this._stateListeners[x] = this._createStateListener();
  			this._editors[x].addEditActionListener(this);
  			this._editors[x].addDocumentStateListener(this._stateListeners[x]);
  		}
  	}
  }

  function _getSelectionController(aWindow) {
  	// display: none iframes don't have a selection controller, see bug 493658
  	if (!aWindow.innerWidth || !aWindow.innerHeight)
  		return null;

  	// Yuck. See bug 138068.
  	let docShell = aWindow.QueryInterface(Ci.nsIInterfaceRequestor)
  		.getInterface(Ci.nsIWebNavigation)
  		.QueryInterface(Ci.nsIDocShell);

  	let controller = docShell.QueryInterface(Ci.nsIInterfaceRequestor)
  		.getInterface(Ci.nsISelectionDisplay)
  		.QueryInterface(Ci.nsISelectionController);
  	return controller;
  }
  
  
  /*******************/
  var doc = gBrowser.contentDocument;
  var searchRange = doc.createRange();
  searchRange.selectNodeContents(doc.documentElement);
  _highlightRange(searchRange, _getSelectionController(gBrowser.contentWindow));

README

Rev1

• Work in progress - does not do anything yet, line 57 throws error obviously

Rev2

• Copy paste working example. Highlights all text in the currently selected tab.
• Beautified the code as well

Rev3

• Updated file name to include snippet as word