Praise be to Allah.
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.