Ten years ago, Tadziu pointed out (http://lists.gnu.org/r/groff/2013-11/msg00016.html) that "afmtodit assumes [the five ligatures groff knows] will be called 'fi', 'fl', 'ff', 'ffi', and 'ffl', but in the new fonts they are called 'f_i', 'f_l', 'f_f', 'f_f_i', and 'f_f_l'."  afmtodit should recognize these alternate ligature names.

In a followup post (https://lists.gnu.org/r/groff/2013-11/msg00025.html), Werner deems expanding afmtodit's default_ligatures hash in this manner a "worse alternative" to doing the ligature redesign called for in bug #64344.  But it also seems a much simpler one, so unless someone is ready to jump on #64344, this afmtodit tweak could be a useful stopgap.