Greasemonkey Flaw Prompts Critical Uninstall Warning By Ryan Naraine, July 19, 2005 http://www.eweek.com/article2/0,1895,1838702,00.asp
"The flaw is so serious that developers are warning users to completely uninstall Greasemonkey versions prior to 0.3.5."
This flaw reportedly can allow any website you visit to see and copy any file on your computer. No exploits have been known to occur yet, but the "how to do it" has just been published. The guy who wrote the plugin says "Get Rid Of It."
I doubt that many are using the Greasemonkey plugin with their Firefox, although the article says it's "popular;" but this does sound rather serious.
The page linked above also contains a link where Firefox users can check up on other recent patches. It's probably an unfortunate fact of life on the web that an occasional check is a good idea.