Security researcher George Hotz, via TippingPoint's Pwn2Own contest, discovered an issue where values are copied from an array into a second, neutered array. This allows for an out-of-bounds write into memory, causing an exploitable crash leading to arbitrary code execution.
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.