YouTube MP4 Downloader works perfectly on Firefox Quantum 57.0 (32-Bit) running under Lubuntu 16.04 LTS (32-Bit). You click on the DOWNLOAD button under the YT video and select the quality you want. Then you can watch the video in an automatically opened separate tab (internal FF video player), right click into this video, choose the "Save As" option, select a directory and save the video. YouTube MP4 Downloader is exactly the solution I need. Thanks for this nice addon!