Is it possible to get the envelope of a signal via FIR filter?
Getting the envelope of a signal requires more than a simple band-pass filter.

However, you could rectify the signal and bandpass-filter the rectified signal to get the enveloppe. That's how I used to do it with ultrasound signals.

https://en.wikipedia.org/wiki/Envelope_detector

You could also use more elaborate techniques, like the Hilbert transform (or Hilbert filter) to get a complex signal that would allow use to use the modulus (or magnitude) of the complex signal to get the envelope.