Per comment:

virtio-net implementation in firecracker does not support VIRTIO_NET_F_MRG_RXBUF. The VIRTIO_NET_F_MRG_RXBUF allows for much finer memory management that the existing one that relies on humongous 68K (17 pages) buffers on guest side. As I understand the 68K buffers also have to be contigous in physical memory and may come hard to achieve under memory shortage pressure.

Please see relevant snipper from virtio spec:
"5.1.6.3.1 Driver Requirements: Setting Up Receive Buffers
If VIRTIO_NET_F_MRG_RXBUF is not negotiated:
If VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6 or VIRTIO_NET_F_GUEST_UFO are negotiated, the driver SHOULD populate the receive queue(s) with buffers of at least 65562 bytes.
Otherwise, the driver SHOULD populate the receive queue(s) with buffers of at least 1526 bytes.
If VIRTIO_NET_F_MRG_RXBUF is negotiated, each buffer MUST be at greater than the size of the struct virtio_net_hdr."