Green’s Dictionary of Slang

(US) a general term of high approval; also used as a magazine name in 1970s–80s.

1995		L. Stavsky et al. A2Z 35/2: firme – adj. (Spanish) first-rate [...] firme heinas – n. (Spanish) hot young girls.
1997		Lil’ Rob ‘Jump in the Ride’ 🎵 Crazy Life [album] Yeah, it’s time for jacking, my homeboys are-a packing / Your fucking carrucha got the fusca so keep trucha / Cuz if you got what we want / If it’s firme as they say and it bumps in your trunk / Then hey man that shit is getting taken.