In Firefox on a Mac, go to the Tools -> Greasemonkey menu and click on one of the scripts to disable it. (i.e. we are trying to remove the tick next to it.)
Then go back to the menu. You will see the script is still enabled!
Confirmed, GM nightly 2012.01.03 and Firefox Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:8.0.1) Gecko/20100101 Firefox/8.0.1.
Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:8.0.1) Gecko/20100101 Firefox/8.0.1
It seems that Firefox on Mac is not sending "click" type events for the main menu bar any more, only "command". Not sure when this changed.
Repair clicking on the main menu, for Mac.