Mozilla developer Johnny Stenback discovered that several methods of a feature used for testing (DOMWindowUtils) are not protected by existing security checks, allowing these methods to be called through script by web pages. This was addressed by adding the existing security checks to these methods.
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.