An anonymous researcher for TippingPoint and the Zero Day Initiative showed that when used in a web page Java would reference properties of the window.navigator object as it started up. If the page replaced the navigator object before starting Java then the browser would crash in a way that could be exploited to run native code supplied by the attacker.
Firefox 1.0 and Mozilla Suite 1.7 are not vulnerable.
Thunderbird does not load Java or other plugin content and is thus not vulnerable.
Disable Java until you upgrade your browser to a version containing the fix.