Seems fw_update(8)'s -p option does something different than
what's needed in this example.

how about this?

Index: faq/faq4.html
===================================================================
RCS file: /cvs/www/faq/faq4.html,v
diff -u -p -r1.559 faq4.html
--- faq/faq4.html	7 Oct 2024 14:20:31 -0000	1.559
+++ faq/faq4.html	23 Dec 2024 21:11:45 -0000
@@ -507,7 +507,11 @@ This example uses firmware for an
 # <b>vnconfig install76.img</b>
 vnd0
 # <b>mount /dev/vnd0a /mnt</b>
-# <b>fw_update -Fv -p /mnt iwm</b>
+# <b>cd /mnt</b>
+# <b>fw_update -Fv iwm</b>
+Get/Verify iwm-firmware-20240410.tgz ... installed.
+fw_update: add iwm; update none
+# <b>cd /</b>
 # <b>umount /mnt</b>
 # <b>vnconfig -u vnd0</b>
 </pre>