Is greasemonkey enabled? If not, the monkey in the firefox status bar
will appear greyed out. If its enabled the install dialog should pop
up automatically when you navigate to the script. If you see script
source when you click it greasemonkey is disabled. You cannot enable/
disable greasemonkey from the manage user scripts/addon preferences
window where the enable checkbox only disables a particular script,
you must enable greasemonkey via one of the menu options for
installation to occur.