Security researcher Ash reported an issue where the extracted files for updates to existing files are not read only during the update process. This allows for the potential replacement or modification of these files during the update process if a malicious application is present on the local system. 


External Reference:

http://www.mozilla.org/security/announce/2014/mfsa2014-16.html


Acknowledgements:

Red Hat would like to thank the Mozilla project for reporting this issue. Upstream acknowledges Security researcher Ash as the original reporter.