If I understand your quesiton correctly you want to avoid the hassle of reinstalling your userscript each time you save it in your editor.
Here are the step-by-step instructions.
Install Greasemonkey in Firefox.
Install your script in Firefox (via Greasemonkey).
Firefox Menu -> Tools -> Greasemonkey -> Manage User Scripts -> Right-click on the script you are working on.
(a) Select "Edit" to open your script in your default editor (emacs?).
or
(b) Select "Show Containing Folder" to open the folder containing your script and subsequently open it with a custom editor (e.g. emacs).
Update the script in your editor as needed and the modification will take effect the moment you save it. Yet you still have to refresh the web page on which your script runs in order to see the result.