If every plugin fails, and you refuse to use command-line tools, use this "analog" Firefox trick:
YouTube frequently updates its frontend code, specifically the JavaScript that generates the "signature cipher" or "signature timestamp" required to access the video stream URLs. A plugin that worked yesterday might fail today because YouTube changed how it generates the token needed to authorize a video stream download. Consequently, the "fix" for a plugin is rarely a one-time repair but rather a constant cycle of reverse engineering. Developers must update their extensions to parse the new YouTube code structures, effectively keeping up with a moving target that is deliberately trying to evade them. youtube playlist downloader firefox plugin fix