Security researcher Masato Kinugawa found a flaw in which the displayed URL values within the addressbar can be spoofed by a page during loading. This allows for phishing attacks where a malicious page can spoof the identify of another site.
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.