The PLUGINSPAGE functionality in Mozilla Firefox before 1.5.0.4 allows remote user-assisted attackers to execute privileged code by tricking a user into installing missing plugins and selecting the "Manual Install" button, then using nested javascript: URLs.  NOTE: the manual install button is used for downloading software from a remote web site, so this issue would not cross privilege boundaries if the user progresses to the point of installing malicious software from the attacker-controlled site.