FreeBSD Bug 195802
According to this: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=195802#c6 the FreeBSD 10.1-RELEASE-p6 has a pretty serious bug with squid dumping core on squid -k shutdown.
On a standard FreeBSD system I can run freebsd-update fetch and install (https://www.freebsd.org/doc/handbook/updating-upgrading-freebsdupdate.html), but these tools aren't available on pfSense (probably for a very good reason).
On stock FreeBSD, you can't freebsd-update to get fixes that aren't in a stable release yet. The associated commit there is only in 10-STABLE. You'd have to svnup stable and build world and kernel.
In our case, we backport fixes along those lines and include them in our releases. 2.2.3 snapshots are available and the fix for 195802 was added today. If you could help test that and report back, it'd be appreciated.