When Private Browsing mode is used, it is possible for a web worker to write persistent data to IndexedDB and fingerprint a user uniquely. IndexedDB should not be available in Private Browsing mode and this stored data will persist across multiple private browsing mode sessions because it is not cleared when exiting.
A combination of an external SVG image referenced on a page and the coloring of anchor links stored within this image can be used to determine which pages a user has in their history. This can allow a malicious website to query user history.
Note: This issue only affects Firefox 57. Earlier releases are not affected.