globalStorage[] working in resource:// - bug or feature?

Hi all,

I've been pining for DOM storage in local (unprivileged) documents for
a while, and looking around (https://bugzilla.mozilla.org/show_bug.cgi?
id=357323, https://bugzilla.mozilla.org/show_bug.cgi?id=469192) things
didn't seem promising.  However, at one point I tried calling
globalStorage['foo'] from a resource://foo/index.html document, and to
my surprised it worked!

Question: is this intended and can it be relied upon, or is it a bug
and will suddenly stop working at some point?

Thanks,
-m