unfollow.js

// Get a list of elements that match selectors, i.e. "Toggle to follow" buttons. 
// Maintainers probably need to change the selector string for new FB versions
var follows = document.querySelectorAll('div[aria-label="Toggle to follow"]');
// If you want to make sure this script doesn't click on the wrong buttons, go
// to the Elements tab, press Ctrl-F, enter "Toggle to follow" in the search
// bar, then find the button that is highlighed

var delay = 1500;  // Delay between clicks to prevent being blocked by FB

var i = 0;  // Initialize a counter of unfollows

function unfollow() {
  // unfollow() calls itself recursively. If counter of unfollows reaches length
  // of friend list, return
  if (i == follows.length) {
    return;
  }

  // Calculate remaining time based on the num. of friends not yet unfollowed
  var remaining = ((follows.length - i) * delay / 1000).toFixed(2);

  console.log(
      'Unfollowing', i + 1, 'of', follows.length + ',', remaining + 's',
      'remaining…');

  follows[i].click();  // Click!

  i = i + 1;  // Increment the counter of unfollows

  // Recursively call itself with a 1500 s delay to keep on unfollowing
  setTimeout(unfollow, delay);
}