Security research firm VUPEN, via TippingPoint's Pwn2Own contest, reported that memory pressure during Garbage Collection could lead to memory corruption of TypeObjects in the JS engine, resulting in an exploitable use-after-free condition.
In general this flaw cannot be exploited through email in the Thunderbird and Seamonkey products because scripting is disabled, but is potentially a risk in browser or browser-like contexts.
Vulmon