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 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.

The bug addressed in Firefox 15.0.1 caused temporary Internet files to be saved inside the browser's disk cache instead of its memory cache when browsing in private mode. Unlike the disk cache, the memory cache is automatically cleared when the browsing session is terminated.

Because temporary files are saved inside the cache together with their original URLs, they can expose what websites users visited while the browser was running in Private Browsing mode.