//since twitter is ajax based, it will only unfollow however people you have loaded. Just refresh the page after each time you run it.

var count = document.getElementsByClassName('user-actions-follow-button js-follow-btn follow-button btn small small-follow-btn').length;

for (i = 0; i < count; i++) { 
   document.getElementsByClassName('user-actions-follow-button js-follow-btn follow-button btn small small-follow-btn')[i].click; 
}