YouTubePlaylistSearch от ItzNotMe

YouTube Playlist Search is a Firefox extension that makes managing and watching YouTube playlists easier.

Search bar
On the Watch page – A search bar appears in the "Add video to..." dialog when you save videos. Quickly find the right playlist without scrolling.
On the Playlists page – A search bar above the playlist grid lets you filter playlists by name while you browse youtube.com/feed/playlists.

Playlist Fix (optional)
When you watch a video inside a playlist, YouTube can autoplay unrelated suggestions. The extension adds an optional Playlist Fix that:

Turns off autoplay for playlist pages
Ensures the next video in the playlist plays when the current one ends
You can enable or disable Playlist Fix anytime from the extension popup.