I have commented on an user's answer to my question [here][1] & I wish to modify it  

but the edit button is missing/doesn't exist  

[![edit button is missing][2]][2]

Why is my comment locked from editing & how can I modify it now or in the future?

  [1]: http://superuser.com/questions/983807/is-there-a-keyboard-shortcut-to-enter-reader-view-in-firefox/1027651#comment1447808_1027651
  [2]: https://i.sstatic.net/LNwM3.jpg