Such properties get their init() method called during first load, getting a handle to the content window, but when reloading a page from the same origin init() is not called again while the content window has changed. See http://mxr.mozilla.org/mozilla-central/source/dom/base/nsDOMClassInfo.cpp#7131 for a relevant comment.
We should probably just fix Bug 690952.
Fabrice, can you make sure your case is fixed now that the fix for bug 690952 got landed? If so, we should mark this fixed.
That works fine, closing this one. Thanks!