In D34445#788774, @cmt wrote:

patch-bug847568 touches old-configure.in - I'd rather ask why it does not regenerate your old-configure (are you even building what I'm building?). If in doubt, avoid any manual manipulation of the source and use poudriere, everything else gives unpredictable results.

In D34445#788389, @cmt wrote:

Turns out this is not enough: the build fails with

Refreshing /wrkdirs/usr/ports/www/firefox/work/firefox-99.0/old-configure with /wrkdirs/usr/ports/www/firefox/work/firefox-99.0/build/autoconf/autoconf.sh
Autoconf requires GNU m4 1.1 or later
ERROR: autoconf exited with return code 1
===>  Script "configure" failed unexpectedly.

That's from ./build/moz.configure/old.configure

The horrors of run-autotools-fixup in bsd.port.mk led me here, no other reason, just seems like a sane thing to do.