I currently use TinyWall Firewall, it works very well, it’s small/portable, no complaints I even donated to the Dev but I would really prefer open source, also it needs to be user friendly like TinyWall so my non-tech family members can/will use it like they do with TinyWall.
I knew I was going to find a comment like this and I am disappointed that I did.
It is hard for people to make transitions specially because they probably used Windows their own life. If they are asking for a FOSS firewall they most likely know they should transition to Linux at some point. There is actually no need to be the questioning person.
I use arch btw