If I understood it correctly, is more like something to run inside firecracker.
Is like a toolkit to build a really small os to run a single application, that then you run on top of a hypervisor.
No, though you could use Firecracker to launch a Mirage (or any other) unikernel.
Basically from the bottom up the stack is:
1. Hypervisor (e.g., KVM, Xen, Hyper-V), runs directly on the hardware
2. Virtual Machine Monitor (e.g., QEMU, Firecracker), running on the host's user-space (say Linux) and in charge of starting/stopping/managing VMs and interacting with the hypervisor
3. Virtual machines, eg, a Linux VM running an NGINX web server.
(the above is simplified because there are differences between type-1 and type-2 hypervisors, but those diffs would make this message too long)
A unikernel is actually a virtual machine, just a very specialized one that doesn't use a general-purpose OS underneath. They tend to use library OSes, so that it's possible to choose libs that are appropriate to each app at build time.
And while we're at it :) , a MicroVM is nothing more than a standard VM (e.g., based on Linux) launched/managed via a fast/modern VMM like Firecracker.
