“A recent thread on the Greasemonkey mailing list suggests that the popular Firefox extension is fatally insecure. It seems rogue pages can read any file from your disk and send it to any site, using an XmlHttpRequest.”
Apparently, installing a new version of the extension solves the insecurities but also causes some old scripts to cease working. As I didn’t have any critical use for GM, I just chose to uninstall it.