Click it to show the best available MP4 quality available for that video. When the add-on is installed, you see a "Download As" button on any YouTube video page, below the "Share" option.
Of all the plug-ins we've tried recently, Easy Youtube Video Downloader Express is the only one that: Many of them are scams or don't work correctly or tell you that you can only download in low quality unless you buy the paid version. There seem to be endless plug-ins and websites that promise to help you download YouTube videos.
The source code is available for anyone to view and audit, so you can be sure that nothing insecure or unexpected happens to the passwords you store.
Make sure to update to the newest version of Firefox because it contains important security updates. Add-ons for earlier versions of Firefox (before version number 57.0) may no longer work with up-to-date versions of Firefox. In 2018, Mozilla made a major update to Firefox called Firefox Quantum.