I am trying to figure a way to allow/deny traffic involving POST requests to any web page using a browser extension.

My biggest problem right now : is there an event to listen for, in order to get all the POST request data, just before it is sent to the server ?

As the target can be any website, it doesn't seem very practical to go for the button listeners, because of the volatility of these components from one website to the other.

I am sorry for the broad aspect of the question.