I'm running the macOS beta for Catalina (10.15) and I'm trying to find out how to create a firm link. (Not a hard link, soft link, alias or anything like that.) The WWDC keynote mentions a new feature referred to as firm links but I can't seem to find out how to do it...
The answer is: you cannot [at least not yet on Betas]
From Apple's official Q/A
Question: Can firmlinks be created by endusers, or are they reserved to the system? If they can be created by the enduser, what commands are used to create them?
Answer: No, firmlinks can't be created by endusers. This is reserved currently to the system. There will be synthetic firmlinks coming, which can be used as mount points for network resources.
Question: If firmlinks can be created by endusers, is there any advantage to using firmlinks over using Unix symlinks?
Answer: Firmlinks can't be created by endusers. Symlinks will be more flexible because they are path-based and able to point to that path regardless of volume ID changes. Firmlinks will be referring to a particular volume.