386BSD -> FreeBSD
NetBSD -> OpenBSD
To some extent I would argue that Linux is a spiritual fork of Minix and Unix.
But I think it’s also worth pointing out that these were in vastly different times where a single person or a small group of people could maintain an OS. An OS or browser is vastly more complex now.
The point which i didnt not mention in my original question is that merging from the fork is a pain, which is why its just easier to jump onto and continuing development on the fork if it gains traction.
Network effects were also nowhere near as strong as they are now.
BSD is a killer example of that point, too. Using a BSD Unix got to be pretty difficult after Linux containers became hegemonic.
It was a bit challenging long before that. I ran webhosting on FreeBSD in the early 2000s - remember finding a tool you needed and hoping it was in ports?