Bash - For loop not working with variables in the declaration

Question

for i in {0..3}
do
   echo $i
done

returns

0
1
2
3

but

firstNumber=$1
lastNumber=$2

for i in {$firstNumber..$lastNumber}
do
   echo $i
done

returns, for example,

{0..3}

What can I do to use variables in a For loop declaration?

Answer 1

for ((i=$firstNumber; i <= $lastNumber; i++)); do echo $i; done