GM_setValue( name, value )
Allows user script authors to persist simple values locally. Strings, booleans, and integers are the only allowed data types.
Values are stored in the Firefox preferences back end and can be changed by typing
about:config in the address bar and searching for the preference name "greasemonkey.scriptval.script-namespace/script-name.
This places limitations on the size of an individual value, and the number of values.
(This statement may not be accurate. Please see this mailing list thread for details.)