Clients/FirefoxExtension/extension/content/overlay.js

var bonjour4firefox = 
{
	onLoad: function()
	{
    	// initialization code
    	this.initialized = true;
    	this.strings = document.getElementById("bonjour4firefox-strings");
  	},
  	onMenuItemCommand: function(e)
	{
    	var promptService = Components.classes["@mozilla.org/embedcomp/prompt-service;1"].getService(Components.interfaces.nsIPromptService);
		promptService.alert(window, this.strings.getString("helloMessageTitle"), this.strings.getString("helloMessage"));
	},
  	onToolbarButtonCommand: function(e)
	{
    	// just reuse the function above.  you can change this, obviously!
    	bonjour4firefox.onMenuItemCommand(e);
	}
};

window.addEventListener("load", function(e) { bonjour4firefox.onLoad(e); }, false);