I've used Windows Firewall for my whole life but I'm curious if anyone has any recommendations for a solid FOSS replacement. I do still use Windows on one of my systems on a daily basis but I always try to replace as much as possible with FOSS alternatives.

Any suggestions for a good FOSS alternative for Windows Firewall (and not at the router level)?

No, "just use Linux" is not a suggestion. I already do use Linux, but there are some things I do have to use Windows for.