Masatoshi Kimura reported a hang caused by a double-free in Thunderbird when processing a large VCard with invalid base64 characters in it. Since an attacker can supply an arbitrary amount of well-formed VCard data before introducing the error we presume this could be exploited to run code of the attacker's choosing.
From the View menu de-select "Display Attachments Inline", and do not open
any VCard attachments (.vcf extension) until you upgrade to a fixed version.
Vulmon