Steven Michaud reported a crash in LiveConnect, the bridge code that allows Java applets and web JavaScript to communicate. The crash is due to re-use of an already-freed object and we presume this could be exploited with enough effort.
Thunderbird does not load Java applets or other plugins in mail messages and is therefore not vulnerable to this flaw unless a user has customized their version of Thunderbird to remove this restriction.
Disable Java to disable LiveConnect until you install a fixed version.