R: dropping all observations within a group

I have a set of 500 firms with investment data over six years. I want to create a balanced panel of ONLY the firms that have data for all six years, but I can't seem to find an easy way to do this in R.

My data looks like:

IMAGE

I want to drop a firm for all years if it has missing values for just one year. I've tried different conditional subsetting commands but nothing is working for me. Ideas?