Etymology tree
Proto-Indo-European *dʰer-
Proto-Indo-European *-mos
Proto-Indo-European *dʰermos
Proto-Italic *θermos
Proto-Italic *fermos
Latin firmus
Proto-Indo-European *-h₂
Proto-Indo-European *-éh₂
Proto-Indo-European *-yéti
Proto-Indo-European *-eh₂yéti
Proto-Italic *-āō
Latin -o
Latin firmoder.
Italian firmarebf.
Italian firma
Back-formation from firmare, from Latin firmāre, from firmus; cognates include English firm (adjective). Doublet of dharma.