Hi Peter,
I try fir.py with MicroPython 1.17 on NUCLEO-L476RG board.
There is a problem only when the number of coefficients is a multiple of 4.
For exemple :
coeffs = array.array('i', (100, 100, 100, 100))
ncoeffs = len(coeffs)
data = array.array('i', [0]*(ncoeffs +3))
data[0] = ncoeffs
data[1] = 0
fir(data, coeffs, 1) # 100 OK
fir(data, coeffs, 0) # 100 OK
fir(data, coeffs, 0) # 100 OK
fir(data, coeffs, 0) # 134492960 bug

coeffs = array.array('i', (0, 100, 100, 100, 100)) works !