CVE-2005-0146

Firefox before 1.0 and Mozilla before 1.7.5 allow remote attackers to obtain sensitive data from the clipboard via Javascript that generates a middle-click event on systems for which a middle-click performs a paste operation...

Synthetic middle-click event can steal clipboard contents — Mozilla

Script-generated middle-click events can steal clipboard contents on systems where that action is a paste. Middle-click paste is the default behavior on Unix systems, and a hidden option elsewhere...