Rated 4 out of 5 stars
I'm not understanding how to use this, I suppose — an earlier version worked differently (though I'd installed my own ffmpeg, and this version insisted on several new installs of ffmpeg, so yes, it's probably user error). But I'm putting the review here (still 4 stars!) because I think there's a new learning curve. I no longer get a video or a song; NOW I get a link from which I can download 2 "video-author-DASH.m4a" files sitting on googleservers? And from that, I have to save and recombine the sound file and video file on my own. (I indeed like having an mp3 or sound file separately, but I used to just strip it on my own.) This is different that the previous version. I'll put time into figuring out what I'm doing, but I knocked a star because of learning curve only. I think I liked the older interface better. (I've waited a while to write this review -- again, not a bug report, but a review of this new version I've downloaded.) I wish I would have kept the OLDER VERSION, so this is a review, and not a bug report.
If you respond that you added options I just need to figure out, or you had to work around (yet another) YT change, that's fine -- believe me, I get it. Regardless, you still get 4 stars at least for the best quality downloads I've tried on several programs/interfaces/OSes (phone apps, Win8 apps, linux apps, other browser plugins) — this is an AMAZING addon. I just liked the older (and to me, simpler) interface better. To me, this is still a donation-worthy add-on even with the interface changes; I find downloading and watching YT videos far preferable to watching them on YT!
If you already have FFmpeg installed on your machine, just go to the settings (Firefox -> add-ons -> YouTube Video and Audio Downloader -> Options) and set "FFmpeg location" to point to your own FFmpeg installation. If you do this, the extension will generate a single video file from those "- DASH" files.
Note that when the extension cannot find the FFmpeg, what it can do is to download video-only and audio-only streams without combining them.