/usr/share/mozilla/extensions/songbird@songbirdnest.com/foxyproxy@eric.h.jung is in xul-ext-foxyproxy-standard 3.4-1.1~deb7u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/mozilla/extensions/songbird@songbirdnest.com/../../../xul-ext/foxyproxy-standard