In certain circumstances persisted XUL attributes are associated with the wrong URL. If an attacker can get a persisted string associated with an URL that will later eval or execute that attribute in a privileged context then the attacker's code will run with the full permissions of the browser.
Thunderbird shares the browser engine with Firefox and could be vulnerable if JavaScript were to be enabled in mail. This is not the default setting and we strongly discourage users from running JavaScript in mail.
Disable JavaScript until you upgrade to the fixed version
Exploit details withheld until sufficient users upgrade to a fixed version