To use it you will need a userscript manager, like Tampermonkey or Greasemonkey.
Creates a button on your playlists which allows you to reverse the order of the videos. Based on the Standalone YouTube Plus Reverse Playlist that is now dead.
I created 1.0.3a as 1.0.3 has stopped working on Firefox and is no longer supported by the creator.
Preview:
###Download
Userscript: ReverseYouTubePlaylist.js
==UserScript==
@version 1.0.3a
@name Reverse Playlist Order for YouTube
@namespace https://github.com/ParticleCore
@description Reverse the order of YouTube playlists
@match ://www.youtube.com/
@run-at document-start
@downloadURL https://github.com/ParticleCore/Particle/raw/master/MDL/rpo.user.js
@grant none
@noframes