How to Port Forward

openwrt says i can do port forwarding by editing /etc/config/firewall file.
but in LibreRouter there is no firewall file in /etc/config

so how do i do port forwarding?

OpenWrt firewall is not installed in LibreRouterOs. I never used it, and AFAIK some LibreMesh developers have issues with it (or its dependencies) but other say it works as expected.
You can directly create iptables rules (yes it is not very convenient) without using any firewall package.