The updated thunderbird packages fix some bugs and security vulnerabilities:
Heap buffer overflow in icalparser.c. (CVE-2019-11703)
Heap buffer overflow in icalvalue.c. (CVE-2019-11704)
Stack buffer overflow in icalrecur.c. (CVE-2019-11705)
Type confusion in icalproperty.c. (CVE-2019-11706)