Iridium by ParticleCore - Iridium is an extension built to improve the user experience on the new YouTube Material layout.
Video DownloadHelper - Browser extension to download videos from the Web
ImprovedTube - Allows you to improve and enhance Youtube visually or enable dozens of new features permanently or...
FlashGot - FlashGot is a browser extension for Mozilla Firefox that is designed to make it easy to download a whole batch of files in one go. The extension is open source and free to use, but it is no longer compatible with the most recent version of Firefox.
Enhancer for YouTube - Tons of features to improve your user experience on YouTube™!
ClipGrab - A friendly downloader for YouTube and other sites