Update Sept 29, 2021:
This developer is the best. He successfully works very hard and quickly to keep this addon working while YouTube frequently changes its code.

Original review from 2018:
For 10 months, I have been using the 'Easy YouTube Downloader Express' in Firefox (currently v61.0.2) on the iMac (macOS Sierra 10.12.6) without any problems. The downloaded video is always good quality. There are several great features this addon sports. Unlike KV 'All Video Downloader' you don't have to go to another webpage with the link to download your video. And unlike Video Download Helper, to select your download format you don't have to sort through a long drop-down list of various codec choices, mixed with lots of jpegs and with other media files. In this Web Extension (aka Addon) the choices for download are consistent from one video to the next. It doesn't require thought or promote confusion. It has downloaded videos the other downloader addons could not. I am very pleased.