URL Envy allows editing URL parameters in the left sidebar, one key/value pair per row. You can edit the keys/values, remove key/value pairs, add new ones, or change the base URL. You can also sort the parameters by key.
URL Envy is based on FireFox's Web Extensions API which is the new standard starting from FireFox v.60. The older style extensions no longer work after this FireFox version. That's why URL Shooter (https://github.com/vmeurisse/url-shooter) by Vincent Meurisse, the extension I used previously for the same purpose, had to be replaced. URL Shooter, in turn, was a replacement for an even older URLParams by Erik Schmidt.
The extension is invoked by pressing Ctrl+Shift+U (on Mac, Command+Shift+U).
