Mozilla released Firefox 15.0.1 on Thursday in order to fix a bug that potentially exposed the websites visited by users while in "Private Browsing" mode.
The goal of the "Private Browsing" mode is to enable Firefox users to surf the Web without leaving any traces of the visited websites behind.
According to a support article
on Mozilla's website, while running in Private Browsing mode the browser shouldn't save visited pages, form and search bar entries, passwords, download entries, cookies, or temporary Internet files, which are collectively known as cached Web content.
The cached Web content consists of images, script files and other resources downloaded automatically by the browser from visited websites. These files are saved and loaded directly from the disk when a Web page is revisited in order to decrease the page's overall loading time.