fwbuilder is not part of the OpenWrt package tree
It does not appear to have been updated since 2013
LEDE is a versatile platform base on GNU/Linux, offering state-of-the art solutions relying on GNU/Linux iptables to filter and inspect packets. Fwbuilder is Free Software allowing to generate complex firewalling rules for iptables, including detailed logging and much more. A detailed feature list of fwBuilder can be found on fwBuilder home page.
This HOWTO explains how to use fwBuilder as an alternative firewalling generator for LEDE, in conjunction with UCI/LuCI firewall.
This article is being written, you are invited to participate if you wish.
fwBuilder GUI is available under the GPL licence under all platforms: GNU/Linux, FreeBSD, Mac OS X and Windows.
Under Debian GNU/Linux, install fwBuilder using these commands:
sudo apt-get install fwbuilder
Do no work on a production firewall while learning fwBuilder.