Firefox plugin that can modify incoming data before the page processes it?

Question

Say a webpage loads an external javascript at load, is there any such FireFox plugin that I could use to modify the javascript before the page actually processes it? (not just specifically javascript)

Thanks in advance. (also I'm pretty sure Tamper Data plugin only changes header data and not actual content being received)

Answer 1

For everyone that has never used tamperdata: Tamper data is for OUTGOING requests. Tamper Data can modify the ENTIRE request, except the URL which requires you to replay the request.

Using GreaseMonkey you can make stand-alone custom plugins that can modify any element of the page before it loads.

Here is THE GUIDE you want which explains GreaseMonkey.

Here are a massive number of GreaseMonkey "UserScripts". This site contains many examples of what you are looking for.

Answer 2

You have a couple options:

1. Tamper Data will modify POST parameters (and GET really since you can modify the URL).
2. You can also combine FoxyProxy (https://addons.mozilla.org/en-US/firefox/addon/2464) with any number of free interactive proxies (Fiddler, Paros, Burp, Charles)
3. Finally you can choose to not use a proxy and write up a greasemonkey script.

I think you'll likely have the most luck with the FoxyProxy + proxy approach. Unfortunately it's not a single addon.

Answer 3

The minimalistic browser-agnostic approach would be to write your own bookmarklet. For example I have found the Show Hiddens bookmarklet to be extremely useful for debugging form submissions. While extremely simple the bookmarklet does things which Tamper Data cannot. I have found it here: http://www.squarefree.com/bookmarklets/forms.html

Also the Forms tab in the Web Developer toolbar has some useful options.