Twitter: Only follow who is following you

twitter_follow.js

// Execute in the javascript console
// Ensure that you scroll to the bottom first

/**
 * Execute in https://twitter.com/following to unfollow all users
 */
$('.profile-stream .following button').click();

/**
 * Execute in https://twitter.com/followers to follow all users
 */
$('.profile-stream .not-following button').click();