All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The prayer that you referred to in the question is not lawful and there is nothing in the Sharee’ah that is called the Firdaws prayer. Rather, what is Islamically permissible to do for a dead person is to perform the funeral prayer for him before burying him, and it is permissible for whoever has not performed the funeral prayer on him to perform it after his burial while standing at his grave. However, the scholars differed in opinion in regard to the time period in which it is permissible to perform the funeral prayer over the grave, some said that it is up to one month and some held a different opinion.
As regards performing a prayer for the dead three days or more or less after his burial contrarily to what we explained above, then this is an innovation that has no evidence in the Sharee’ah and the Prophet said: “Whoever performs an act which is not in accordance with our matter [religion], it will be rejected from him.” [Al-Bukhari and Muslim]
Allaah Knows best.