Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
firmness [ˈfəːmnɪs] n (of ground, decision, faith) твёрдость f (of mattress) жёсткость f (of grip, hold) кре́пость f
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.
Русский: