Despite the name, it has a dedicated "Extract Audio" mode. You just paste the link, choose MP3, and it handles the rest.
This is one of the most popular "cross-browser" plugins. It adds a dedicated "Download" button below the YouTube video player, allowing you to select MP3 (128kbps, 256kbps, or 320kbps) directly. skachat s youtube mp3 plagin
Avoid any plugin that asks for excessive permissions (like "reading your data on all websites"). Despite the name, it has a dedicated "Extract Audio" mode