Mozilla developer Bobby Holley discovered that it was possible to bypass some protections in Chrome Object Wrappers (COW) and System Only Wrappers (SOW), making their prototypes mutable by web content. This could be used leak information from chrome objects and possibly allow for arbitrary code execution.
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.