I think I understand the use case for firecracker if you're a hosting provider like AWS or Fly.io: you don't want one customer's containers messing with those of another container.
If you're only running your own code, is Firecracker better in some way than just using docker?
It offers a greater degree of sandboxing
Right, but if I'm only running my own code, I probably don't care about that, right?
Depends, is your code being served to the internet? If it is I would definitely consider firecracker but if its just internal it probably won't matter a whole lot. Not sure about the overhead of docker compared with firecracker though, so there may be advantages there either way.
There's also ignite which works with firecracker: https://github.com/weaveworks/ignite