The latest firewalld (and I think the old version Solus currently packages) are unable to start because the kernel is missing the config option CONFIG_NFT_FIB_INET=m. Without it, firewalld fails to start with cryptic "file or directory not found" errors when it tries to add its ruleset.

I verified this by enabling the kernel config option for it, building the kernel, and seeing that firewalld works after rebooting.