See comments section for more up-to-date versions of the script. The original script is from 2014 and will not work as is.
- Open news feed preferences on your Facebook menu (browser)
- Click people or pages
- Scroll down (or click see more) until your full list is loaded
- Run the script in your browser console
Facebook will block this feature for you while you use it, depending on how much entities you try to unfollow. It automatically unblocks in a couple of hours and you will be able to continue.
Are you asking about "unfollowing" a "friend"? Or, are you asking about "unfriending" a "friend"?
For the latter, I used L.O.C. browser extension with good results, which includes a "Friends Remover: find and remove friends with ease."
It gave me a list of all my friends, and I was able to check one box to mark 100 at a time and then click once to remove all 100. I had 400-600 friends, so it only took about 15 clicks to "unfriend" all of them. Note it did take a few minutes to process each batch of 100, so total time was several minutes, but effort was very low.
For the former (un_following_ other people), no, that I'm not familiar with.