How to use the BeagleBone's PRU with a modern kernel?
The /lib/firmware/ is simply where you place the binaries that you compile for the PRU. Your PRU code will end up here so that it can be loaded by the PRU kernel module.

Old versions of the TI rproc driver require the name to be am335x-pru0-fw, however in more recent version this requirement has been relaxed (although it's still convention).