Messpostage

MessPostage

Notifies you if a site uses the postMessage API. You can install this extension from its Firefox Add-On page.

This extension puts a little envelope in your toolbar. As soon as the site you are visiting adds a message event listener or posts a message, it becomes red:

<img src="screenshots/toolbar.png">

Clicking this button will shows which messages were sent and where event listeners have been added:

<img src="screenshots/popup.png">