Looks like our FIRRTL files for BOOM are older than current Firtool supports:
$ make -C boom run
firtool --ir-hw build/small/boom.fir -o build/small/boom.mlir
build/small/boom.fir:1:16: error: FIRRTL version must be >=2.0.0
FIRRTL version 1.1.0
^
make: *** [Makefile:35: build/small/boom.mlir] Error 1