Stability
Will your chosen firewall still be here 10 years from now?
If sustainability is what drives you, you can do far worse than PFire. It was originally developed in 2006 as a fork I of IPCop, and nearly 20 years later is still going strong. The project is a member of the Open Invention Network, which champions the use of open source software. This is hardly surprising given that IPFire itself is free and open source software, meaning others are free to fork the project’s work if it stalls for any reason.