1. First, you need to know that m (or so-called a) denotes the number of bands.
That is, m = 3 in your case since you have two stopbands and one passband.
2. Second, f is the edge frequency vecor that inherently removes the starting and
end points. Thus, 0 and pi in your f are redundant and should be removed.
3. Third, dev corresponds to m. Hence, your dev should have only 3 elements.
4. Last, f and Fs should have the same units. Use either radians or Hz.
So, under Fs = 16000 Hz, your codes should be modified as follows.
Fs=16000;
m=[0 1 0];
f=[0.2*Fs/2 0.3*Fs/2 0.5*Fs/2 0.6*Fs/2];
dev=[0.05 0.01 0.05];
Fs=16000;
[n,fo,mo,w]=firpmord(f,m,dev,Fs);
h = firpm(n,fo,mo,w);
figure,freqz(h,1,1024,Fs);
Hope it helps! ^^
