who has the metarial of FIR

Now I'm really confused. It sounds as if you want to apply an FIR to the data before applying an FFT to it - is that right?

An FIR is just a convolution on the data stream. If you are already doing an FFT any way, it's normally easier to use the Convolution Theorem and apply the convolution after the FFT, rather than before.

What is the specific reason that makes you want to do the FIR before doing the FFT, rather than after it?

If you want to do an FIR instead of an FFT, then my original questions have not been answered: What is is you want to know about FIRs? What are you trying to do with them? Since every application is unique and different, and since there are so many possible complications in implementing FIRs, I can not offer any suggestions until I have more information about what you are trying to do.