Where does “*firmāc(u)lum” come from?
*firmāc(u)lum (Vulgar Latin) comes from Latin firmāre, from Latin Firmus, from Proto-Italic fermos, from Proto-Indo-European dʰer-mo-s, from Proto-Indo-European dʰer- — to support; to hold.
Ancestry of “*firmāc(u)lum”, step by step