firefox dumping core after NetBSD upgrade

[Paul Goyette <paul%whooppee.com@localhost>]

I recently upgraded my system from 9.99.4 to 9.99.15, and I've noticed
that firefox is repeatedly dumping core.  I haven't found the specific
trigger yet, but...

* The host system is a NetBSD amd64
* firefox 69.0.1 (and all of its dependencies) was build from pkgsrc,
  while the host was still running 9.99.4

I don't have debug symbols available, but I was able to get a stack
back-trace using gdb;  hopefully someone might recognize what the
problem is...

Core was generated by `firefox'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x00007af20d009a7b in _atomic_cas_ptr (new=<optimized out>, old=0x0,
    ptr=0x7af20d3cb730)
    at /build/netbsd-local/src_ro/lib/libpthread/arch/x86_64/pthread_md.h:77
77              __asm __volatile ("lock; cmpxchgq %2, %1"
[Current thread is 1 (process 1)]
(gdb)
(gdb) bt
#0  0x00007af20d009a7b in _atomic_cas_ptr (new=<optimized out>, old=0x0,
    ptr=0x7af20d3cb730)
    at /build/netbsd-local/src_ro/lib/libpthread/arch/x86_64/pthread_md.h:77
#1  pthread_mutex_lock (ptm=ptm@entry=0x7af20d3cb720)
    at /build/netbsd-local/src_ro/lib/libpthread/pthread_mutex.c:201
#2  0x00007af1ec643f74 in mtx_lock (mtx=0x7af20d3cb720)
    at /build/netbsd-local/xsrc_ro/external/mit/MesaLib/dist/include/c11/threads_posix.h:223
#3  simple_mtx_lock (mtx=0x7af20d3cb720)
    at /build/netbsd-local/xsrc_ro/external/mit/MesaLib/dist/src/util/simple_mtx.h:125
#4  _mesa_error (ctx=ctx@entry=0x7af20d3b9868, error=error@entry=1282,
    fmtString=fmtString@entry=0x7af1ee415f84 "Inside glBegin/glEnd")
    at /build/netbsd-local/xsrc_ro/external/mit/MesaLib/dist/src/mesa/main/errors.c:309
#5  0x00007af1ec65e256 in _mesa_GetString (name=7938)
    at /build/netbsd-local/xsrc_ro/external/mit/MesaLib/dist/src/mesa/main/getstring.c:124
#6  0x00007af1fbdd534d in ?? () from /usr/pkg/lib/firefox/libxul.so
#7  0x00007af1fbdd5828 in ?? () from /usr/pkg/lib/firefox/libxul.so
#8  0x00007af1fbdcb6a4 in ?? () from /usr/pkg/lib/firefox/libxul.so
#9  0x00007af1fbdd2df8 in ?? () from /usr/pkg/lib/firefox/libxul.so
#10 0x00007af1fbdd3649 in ?? () from /usr/pkg/lib/firefox/libxul.so
#11 0x000000012e805f57 in _start ()
(gdb)

+--------------------+--------------------------+-----------------------+
| Paul Goyette       | PGP Key fingerprint:     | E-mail addresses:     |
| (Retired)          | FA29 0E3B 35AF E8AE 6651 | paul%whooppee.com@localhost     |
| Software Developer | 0786 F758 55DE 53BA 7731 | pgoyette%netbsd.org@localhost   |
+--------------------+--------------------------+-----------------------+