None of those is actually the “pkgconfig”. For example,
dnf provides 'pkgconfig(nss)'
dnf rq --whatprovides 'pkgconfig(nss)'
will both tell that feature “pkgconfig(nss)” is provided by package nss-devel.
That feature is info for the build tools on how to include/link the nss library.
Obviously, the build tools do use some utility to read the info and that is the pkgconfig.
One could ask whether there is a file with that name in some package:
dnf provides */pkgconfig
but that output does not make us wiser.
One could ask whether such feature is provided by some packages:
dnf provides pkgconfig
dnf rq --whatprovides pkgconfig
However, one can also ask what the nss-devel does require:
dnf rq --requires nss-devel
dnf rq --requires --resolve nss-devel
Therefore, the installation of nss-devel (or probably any of the needed packages) will auto-install the “pgkconfig” too.
Why are you building Firefox? Rocky does provide one?
Are you building with rpmbuild?
The mock supposedly auto-installs those required packages (and does it in “clean” chroot).