Firefox plugin -- 2

We want to intercept the data from a certain browser page and write some of the data to a local file before the page is submitted. We have users that will enter data in to a 3rd party web site and then print a button to save the data but we want to write out the Invoice number they have entered and write it to disk so we can pick it up and process it using an existing application we have. We need a Firefox plugin that can detect the page we are looking for, intercept the save button event then read the data from the DOM and write it out locally then process the original Save event.