Mozilla developer Boris Zbarsky discovered an issue where network-level redirects cause an <iframe> sandbox to forget its unique origin and behave as if the allow-same-origin keyword were applied. This allows the sandboxed content to access other content from the same origin without explicit approval.
In general this flaw cannot be exploited through email in the Thunderbird product because scripting is disabled, but is potentially a risk in browser or browser-like contexts.
Red Hat would like to thank the Mozilla project for reporting this issue. Upstream acknowledges Boris Zbarsky as the original reporter.
This issue does not affect the version of thunderbird as shipped with Red Hat Enterprise Linux 5 and 6, or the version of firefox as shipped with Red Hat Enterprise Linux 5, 6, and 7.