Mozilla developer Jeff Gilbert discovered a mechanism where a malicious site with WebGL content could inject content from its context to that of another site's WebGL context, causing the second site to replace textures and similar content. This cannot be used to steal data but could be used to render arbitrary content in these limited circumstances.
In general this flaw cannot be exploited through email in the Seamonkey product because WebGL is not enabled, but is potentially a risk in browser or browser-like contexts.
Vulmon