This happens with firefox from upgrade to upgrade. Sometimes more stable, sometimes crashes continuously. Characteristic of bloat software: it does not evolve, only mutate and get fatter.
Code:
# uname -a
FreeBSD fbsd.local 11.2-RELEASE FreeBSD 11.2-RELEASE #0 r335510: Fri Jun 22 04:32:14 UTC 2018 root@releng2.nyi.freebsd.org:/usr/obj/usr/src/sys/GENERIC amd64
# firefox --version
Mozilla Firefox 64.0.2