It is a addon for firefox that stops you from accessing programs with coding. To test this, go to notepad and type:
@echo off shutdown.exe -s -t 10 "Please wait, deleting GREASEmonkey addon"
It will block you from going there, because it detects code.
If you go on McAfee it should detect it as malware, but if it doesn't, click the button on the top right, click addons, click GREASEmonkey, click remove.