A serious security flaw discovered in the Greasemonkey Firefox extension requires all users to updowngrade to version 0.3.5, a stripped-down version of the extension, available for download. This means some GM scripts that used advanced functionality will not work while the GM developers patch the flaw, but the one posted here for saving posts to Yahoo!'s MyWeb works fine with the "neutered version" update.
Update: Developer Simon Willison describes the details of the Greasemonkey flaw clearly and calmly.
Mandatory Greasemonkey Update [Greaseblog]