Remijia firmula is characterized by its usually dense pilosulous to strigose pubescence, medium-sized (in Remijia) opposite petiolate leaves, rather well developed, generally racemiform inflorescences, medium-sized flowers with the calyx limb lobed with acute lobes, and capsules that generally open from the base. This is perhaps the most commonly collected species of Remijia.
The plants included in Remijia firmula at present show a notable variation in leaf texture, from chartaceous to subcoriaceous, size, number of secondary veins, and pubescence, from densely pubescent with short to rather long trichomes to glabrescent; the species probably deserves re-evaluation now that more material is available. In any case rRemijia firmula as currently identified in herbarium material includes plants that have lanceoloid capsules that open from the base, which probably correspond also to the plants described Roraima cinchonicarpa but may not correspond to the plants originally described as Remijia firmula. The only plants referrable in general to Remijia firmula so far seen from southern Colombia have stouter, cylindrical capsules that open from the apex. Martius's type was collected at Araracuara in southern Colombia, and is in flower. Later Schumann (1889) included a second collection, Spruce 2954, in the circumscription of this species and the Spruce specimen does have slender lanceoloid capsules that open from the base but whether this is the same as the plants Martius originally collected remains to be confirmed. Steyermark (1972) characterized Remijia firmula in part by its slender capsules, so these plants apparently fall within his circumscription of Remijia firmula.
Remijia firmula is similar to Remijia roraimae, which has subtruncate to shallowly broadly lobed calyx limbs and is generally found at higher elevations; herbarium specimens of these can sometimes be difficult to separate with confidence. Remijia roraimae var. adpressa differs from the other plants included in that species in its lower elevation range and larger, rather sparsely pubescent leaves. Remijia firmula is also similar to Remijia longifolia; these species were separated by Steyermark (1972) based on the presence in Remijia firmula vs. absence in Remijia longifolia of glands (colleters) in the sinuses of the lobes of the calyx limb, however this feature varies in many Rubiaceae and probably deserves further evaluation. These were also separated by sessile flowers, stouter capsules, and a style pubescent at the base in Remijia longifolia vs. pedicellate, more slender, and glabrous, respectively, in Remijia firmula. In the descriptions of these they are also separated by their leaf arrangement, ternate in Remijia longifolia vs. opposite in Remijia firmula (Steyermark, 1974), however at least one Venezuelan specimen identified as Remijia longifolia by Steyermark has opposite leaves. These two species were separated by Taylor (2004) based on acute leaf apices in Remijia firmula vs. truncate leaf apices in Remijia longifolia, but that was an error, both of these species have acute to acuminate leaf apices; here Remijia longifolia was also characterized as having opposite leaves based on herbarium identifications, but that seems also be inaccurate. The separation of Remijia firmula, Remijia roraimae, and Remijia longifolia thus probably deserve more study.