Advisory Text :  

There is an interesting vulnerability in how Mozilla Firefox handles
internal wyciwyg:// pseudo-URIs. These cache-related resource
identifiers
are meant to be inaccessible by the user - but there are at least three
routes to bypass these restrictionss, one of which - HTTP 302 redirect
-
also improperly employs same-domain policy checks.

This combo flaw enables attackers to intercept sensitive data, perform
cache poisoning, or carry out URL spoofing (including SSL certs),
against
sites that scriptually render documents on client side, and hence
produce
wyciwyg:// resources to begin with. Although not all sites are
susceptible
to attacks, a good chunk of "Web 2.0", a selection of popular webmails,
and several major banks, very much are.

A quick demo and a more detailed discussion can be found here:

http://lcamtuf.coredump.cx/ffcache/

PS. The two remaining routes to bypass wyciwyg:// restrictions
(XMLHttpRequest() and view-source: URIs) appear to properly implement
same-domain checks (although view-source seems to be nevertheless not
functioning as intended). document.write() + XMLHttpRequest to
wyciwyg://
URIs can be used by rogue websites to conveniently store and retrieve
persistent "markers" on visitor's machine regardless of cookie
settings;
that's not a disaster, but still not very nice.

PS2. Bugzilla entry here - source patch available:
https://bugzilla.mozilla.org/show_bug.cgi?id=387333

Cheers!
/mz