9.99.86 system lockup opening Firefox's about:support page

[RVP <rvp%SDF.ORG@localhost>]

Upgraded the kernel to 9.99.86 hoping to fix this issue:
https://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=56285

but, now, opening `about:support' in Firefox-86.0.1 (from 2021Q1)
completely locks-up the system. This is the case with the intel
Xorg driver (both SNA & UXA) and, surprisingly, also genfb+wsfb.

Anybody else have this issue?

The kernel is:

$ uname -a
NetBSD x202e.localdomain 9.99.86 NetBSD 9.99.86 (MYKERNEL) #0: Fri Jul  2 
21:33:28 UTC 2021 
bld@x202e.localdomain:/usr/obj/usr/src/sys/arch/amd64/compile/MYKERNEL 
amd64

which is just the very latest-from-CVS GENERIC compiled as MYKERNEL.

Video card is:

$ fgrep 'HD ' /var/log/Xorg.0.log
[    57.760] (II) intel: Driver for Intel(R) HD Graphics
[    57.770] (--) intel(0): Integrated Graphics Chipset: Intel(R) HD Graphics 4000

Userland and Xorg are also the latest prebuilt ones: 202107021200Z.

-RVP

PS. BTW, there are no crash dumps--the system just hangs solid.