By default YouTube sorts the display of playlists by usage. This Firefox extension overrides that behaviour to be alphabetical (A-Z) in both the main sidebar, and the selection modal that is brought up when deciding which playlist(s) to add a video to.
Note
While a few Google Chrome extensions exist to support this behaviour, there were none for Firefox and such requests seemingly fell on deaf ears. This extension has been created primarily for my own use case and support/future changes (e.g. fixing in the event of a complete design overhaul) may or may not happen.
This project is licensed under the MIT License.