You can alter their values, add new parameters, switch get/post and more.
For more info and how to use UrlParams, have a look at http://urlparams.blogwart.com
FF7-9 version available at the homepage.
For a recent version go to the addons homepage. (Please be sure to read changelog) - pseudo-update for FF7-9 available. Nothing fixed, yet. Close and re-open Urlparams for new Tabs.