It seems that v 15 failed to operate adequately in private browsing mode, with files being saved to disk, as opposed to being in memory only.
This new version resolves the issue but it doesn't clear the files and cookies you may have downloaded while in private browsing mode.
How to clear Firefox cache. [support.mozilla.org]
Firefox v.15.0 Released [webmasterworld.com]
Firefox 15.0.1 Fixes Not So Private Browsing Bug
