Security researcher regenrecht reported, via TippingPoint's Zero Day Initiative, a use-after-free in XMLSerializer by the exposing of serializeToStream to web content. This can lead to arbitrary code execution when exploited.
In general these flaws cannot be exploited through email in the Thunderbird and SeaMonkey products because scripting is disabled, but are potentially a risk in browser or browser-like contexts in those products.
Vulmon