Firefox - crashes when using the "close tab" button - sometimes

Here is an annoying bug in Firefox: when I click on the "close tab" button, sometimes Firefox crashes (or performs an unclean shutdown, because on restart it asks to restore windows and tabs). Has anyone else seen this bug?
- discovered after I upgraded Firefox to 51.0.1_2,1 (I'm pretty sure this bug was not present in earlier versions)
- if I use Ctrl-W to close a tab Firefox never crashes (not true anymore, so far I have one crash while using Ctrl-W)
- this crash / unclean shutdown never produces a core dump
tingo@kg-core1$ uname -a
FreeBSD 10.3-STABLE FreeBSD 10.3-STABLE #0 r310083: Wed Dec 14 21:00:13 CET 2016  amd64
root@kg-core1# grep pkg /var/log/messages | grep firefox
Feb 21 23:03:50 kg-core1 pkg: firefox-50.1.0_4,1 deinstalled
Feb 21 23:03:54 kg-core1 pkg-static: firefox-51.0.1_2,1 installed
Oh, and I haven't upgraded Firefox to 52.0,1 yet because of the plugin removal, I need to verify that I don't use java or other plugins for anything first.
And just to prove that Murphy hasn't forgot about us, Firefox just now crashed when I used Ctrl-W. I'll update the first post. Sigh.
The problem also happens on another FreeBSD 10.3 machine, this one updated via freebsd-update, and only packages installed.
root@kg-quiet# freebsd-version -ku
root@kg-quiet# uname -a
FreeBSD 10.3-RELEASE-p11 FreeBSD 10.3-RELEASE-p11 #0: Mon Oct 24 18:49:24 UTC 2016  amd64

root@kg-quiet# pkg version -n firefox
Updating FreeBSD repository catalogue...
FreeBSD repository is up-to-date.
All repositories are up-to-date.
firefox-51.0.1,1                   =
I'm a bit annoyed by this problem. :-/