Script injection firefox

PostUtils.executeScript("chrome://extension/content/contentscript.js", "onSelectionReceived", onSelection);
executeScript: function(script, messageKey, callback) {
        var wm = Components.classes["@mozilla.org/appshell/window-mediator;1"].getService(Components.interfaces.nsIWindowMediator);
        var mainWindow = wm.getMostRecentWindow("navigator:browser");
        Components.utils.reportError("loadContentScript() called for " + script);
        mainWindow.gBrowser.selectedBrowser.messageManager.loadFrameScript(script, true);
        mainWindow.gBrowser.selectedBrowser.messageManager.addMessageListener(messageKey, callback);
        Components.utils.reportError("loadContentScript() executed for " + script);
    },