As documented in https://fedoraproject.org/wiki/Using_the_Koji_build_system#Initial_Fedora_Setup, simply running fedora-packager-setup and following the instructions to import the certificate into Firefox should enable one to login to Koji. However Firefox, says:
Secure Connection Failed koji.fedoraproject.org uses an invalid security certificate. The certificate is not trusted because the issuer certificate is not trusted. (Error code: sec_error_untrusted_issuer)
fedora's koji's use self signed certificates. the ca cert is available from https://admin.fedoraproject.org/fingerprints
you can download and import the CA cert. it will then work for the primary and secondary arch koji hubs. either way this is not a fedora-packager issue.