I didn't know it existed until they posted, but QEMU has a Firecracker-inspired target:
> microvm is a machine type inspired by Firecracker and constructed after its machine model.
> It’s a minimalist machine type without PCI nor ACPI support, designed for short-lived guests. microvm also establishes a baseline for benchmarking and optimizing both QEMU and guest operating systems, since it is optimized for both boot time and footprint.