Greasemonkey scripts can save data for reuse. Uninstallation can keep your saved data or erase it.
There are two type of uninstallation:
1. Soft Uninstall
2. Clean uninstall
A soft uninstall only removes the script from your list of scripts so it won't run on any pages.
A clean uninstall also removes the saved data (associated preferences) that go with the script.
In Firefox window go to Tools->Greasemonkey->Manage User Scripts like this:
Click on Manage User Script and you will see a box like this:
Soft Uninstall
Click on the script you want to remove in the left column.
Click on Uninstall.
Clean Uninstall
Click on the script you want to remove in the left column.
Check the Also uninstall Associated preferences box at the bottom.
Click on Uninstall.