Extension is working for me but installation is clunky and instructions for downloading are so minimal that it took me a couple of hours of experimentation to figure out that it won't merge separate video and audio together even though some of the "documentation" implies that it can. To get a working video with audio, you must download the very first file on offer.

If you want just video without audio, that's clunky too. I try to download the vid links and instead of just one file to save it pops up two. Both have same filename, so if you choose to save the 2nd as well as the first, you obviously overwrite the 2nd (this extension will start saving both, but as soon as the smaller file is done downloading, it fails and simultaneously cancels the other download, so you get nothing). If you cancel the second, a bunch more files pop up. If you keep cancelling these subsequent files until there aren't any more, you'll get just the audio file, no video. If you want the video, you're hosed. If choose to cancel the first file and save the second (and cancel any subsequent files that pop up after the second one), finally you get video only, no audio. But getting to that point is very confusing. What's with all the extra files popping up one after the other, each in their own download dialog?

Getting an audio-only file is, thankfully, just the way it should be. Click on the mp4 audio-only file (if available) and it just gives you a single file to download, exactly the way it should. (I haven't tried webm-opus, so can't speak to that option).