The basic principle with regard to naming children is that it is permissible to give any name so long as it is not forbidden in sharee‘ah.
It says in al-Mawsoo‘ah al-Fiqhiyyah (11/331):
The basic principle is that it is permissible to give any name except that which it is narrated is forbidden. End quote.
There is no evidence – as far as we know – that calling a girl Firdaws or Jannah is not allowed. So there is nothing wrong with using these names.
These are names that carry good and beautiful meanings.
Firdaws in Arabic refers to a garden in which all kinds of plants grow, and Jannah is a garden in which there are palm trees and other kinds of trees.
See: al-Qamoos al-Muheet, p. 725, 1532; Tahdheeb al-Lughah by al-Azhari, 13/104, 105.
And Allah knows best.