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for i in fw[0-9][0-9]
do
mv $i "fw0${i: -2}"
done

Try:
for f in fw??; do
echo mv "$f" "fw$(printf "%03d" ${f#fw})"
done

remove echo if it looks OK...

The mask is optional, if you don't give one it doesn't mask anything out (or assumes /32, however you want to describe it).