Firefox before 1.0 does not properly distinguish between user-generated and synthetic click events, which allows remote attackers to use Javascript to bypass the file download prompt when the user uses the Alt-click feature.
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A100051
https://exchange.xforce.ibmcloud.com/vulnerabilities/19170
https://bugzilla.mozilla.org/show_bug.cgi?id=265176