UrlParams, a Mozilla add-on, shows the current website's GET and POST parameters.
One of the great features of UrlParams is the ability to modify the parameters shown in the sidebar which ultimately means you can alter their values easily. You can also add new parameters or switch between GET and POST depending on your preference.
The software has a straightforward system of requirements which means in order to use UrlParams, you'll need Firefox, and nothing more. This software is precisely what you need if you want to have a quick and easy view of a website's GET and POST parameters.
Version 2.2.0: N/A