Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
firmātor, ōris, m. [firmo], a confirmer, establisher (post-Aug. and very rare): missus Drusus paci firmator (so acc. to Cod. Med., not pacis), Tac. A. 2, 46: disciplinae militaris, Plin. Ep. 10, 38, 1.