Look here for some help.
Especially this comment near the bottom:
I may have fixed this. I did two things, I created an empty text file named "config.xml" and placed it in every directory under: C:\Users\VISTA_USER_NAME\AppData?\Roaming\Mozilla\Firefox\Profiles \ljnuw7ej.default\extensions\{e4a8a97b-f2ed-450b-b12d-ee082ba24781} That may be different on your system, so open the profiles folder and do a search for "grease", use that path.
I also had two folders that I had not seen before that started with "gm_" located at: C:\Users\VISTA_USER_NAME\AppData?\Roaming\Mozilla\Firefox\Profiles \ljnuw7ej.default\ I moved them to a temporary location rather than delete them just in case. Once I did those two things, Greasemonkey is working again!
Replace VISTA_USER_NAME with the name you log into Windows with in the paths above.
Not sure if your case is the same, but worth trying.