Firmicus is a lunar crater that lies in the eastern part of the Moon's near side, so that from Earth it appears oval in shape due to foreshortening. It is, however, very nearly circular. The crater is located to the west of the Mare Undarum, and northeast of the similar-sized crater Apollonius. To the north of Firmicus are the craters van Albada and Auzout. Attached to its northwest rim is the  Lacus Perseverantiae, a miniature lunar mare.