How to access webpage data from a firefox extension?

Question

I have managed to get a custom very basic extension running in Firefox.

What I want to do next is:

1. Check if the user is on a desired webpage

Answer 1

So what is leftover for you is the DOM traversal and the external program launching.

Your DOM traversal can be done in so many ways. However, here is a simple take

var inputs = document.getElementsByTagName("input");
for (var idx=0; idx<inputs.length; idx++){
    var tp = inputs[idx].attributes['type'].value
    console.log(tp);
    if (tp == 'hidden'){
       // grab your text at here and launch the app.
    }
}

External application launching according to this post

var file = Components.classes["@mozilla.org/file/local;1"]
                     .createInstance(Components.interfaces.nsILocalFile);
file.initWithPath("c:\\myapp.exe");
file.launch();