browser or proxy hack that would modify website files before browser parses/loads them

I know how to make browser automation tools that work with the website in browser after it fully loaded. But, what I want here is being able to modify the website files BEFORE they loaded. This applies particularly to the Javascript files, but if possible, let's do it for both the html and javascript. Here is a simple example. Suppose we have a website that contains a javascript function that executes on load of the webpage. So using my current DOM modification techniques if I try messing with the DOM after loading, it will be too late - the Javascript would have already executed. So, in this case I would like to be able to grab the Javascript file after it got downloaded to my machine over the network but before it got loaded/parsed/executed by the browser and modify it at will, and then feed it back into the pipeline. E.g. with the offending function cut out. I am guessing that something like that could be achieved using either a firewall style proxy or using a browser plugin. Well, not sure what is an easy way to do it, but please tell me how you intend to go about it. ## Deliverables 1) Complete and fully-functional working program(s) in executable form as well as complete source code of all work done. 2) Deliverables must be in ready-to-run condition, as follows (depending on the nature of the deliverables): a) For web sites or other server-side deliverables intended to only ever exist in one place in the Buyer's environment--Deliverables must be installed by the Seller in ready-to-run condition in the Buyer's environment. b) For all others including desktop software or software the buyer intends to distribute: A software installation package that will install the software in ready-to-run condition on the platform(s) specified in this bid request. 3) All deliverables will be considered "work made for hire" under U.S. Copyright law. Buyer will receive exclusive and complete copyrights to all work purchased. (No GPL, GNU, 3rd party components, etc. unless all copyright ramifications are explained AND AGREED TO by the buyer on the site per the coder's Seller Legal Agreement). ## Platform Windows, Javascript, proxy, Mozilla, IE, browser automation