Mozilla developer Boris Zbarsky reported that user-defined
getters on DOM proxies would incorrectly get the expando object as this
.
It is unlikely that this is directly exploitable but could lead to JavaScript
client or add-on code making incorrect security sensitive decisions based
on hacker supplied values.
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.