Mozilla security researcher moz_bug_r_a4 reported a method to use browser navigations through history to load a website with that page's baseURI property pointing to that of another site instead of the seemingly loaded one. The user will continue to see the incorrect site in the addressbar of the browser. This allows for a cross-site scripting (XSS) attack or the theft of data through a phishing attack.
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.