FirefoxがQuantumになる前までは、簡単にできましたが、Pluginがなかなかなく。 コマンドラインのツール。

https://rg3.github.io/youtube-dl/

下記のコマンドたたくと、こんな感じでダウンロード出来るフォーマットが出てきます。

PS F:\YouTubeDownload> .\youtube-dl.exe --list-formats https://www.youtube.com/watch?v=Zziq8qY6wko
[youtube] Zziq8qY6wko: Downloading webpage
[youtube] Zziq8qY6wko: Downloading video info webpage
[youtube] Zziq8qY6wko: Extracting video information
[youtube] Zziq8qY6wko: Downloading js player vflfv8a8v
[info] Available formats for Zziq8qY6wko:
format code extension resolution note
249 webm audio only DASH audio 55k , opus @ 50k, 1.18MiB
250 webm audio only DASH audio 78k , opus @ 70k, 1.61MiB
140 m4a audio only DASH audio 128k , m4a_dash container, mp4a.40.2@128k, 2.84MiB
171 webm audio only DASH audio 140k , vorbis@128k, 2.72MiB
251 webm audio only DASH audio 151k , opus @160k, 3.13MiB
278 webm 256x144 144p 108k , webm container, vp9, 24fps, video only, 2.23MiB
160 mp4 256x144 144p 113k , avc1.4d400c, 24fps, video only, 2.46MiB
242 webm 426x240 240p 247k , vp9, 24fps, video only, 4.64MiB
133 mp4 426x240 240p 250k , avc1.4d4015, 24fps, video only, 5.44MiB
243 webm 640x360 360p 447k , vp9, 24fps, video only, 8.23MiB
134 mp4 640x360 360p 489k , avc1.4d401e, 24fps, video only, 7.26MiB
244 webm 854x480 480p 765k , vp9, 24fps, video only, 13.47MiB
135 mp4 854x480 480p 909k , avc1.4d401e, 24fps, video only, 13.78MiB
247 webm 1280x720 720p 1483k , vp9, 24fps, video only, 25.44MiB
136 mp4 1280x720 720p 1599k , avc1.4d401f, 24fps, video only, 24.85MiB
248 webm 1920x1080 1080p 2551k , vp9, 24fps, video only, 44.34MiB
137 mp4 1920x1080 1080p 2780k , avc1.640028, 24fps, video only, 44.84MiB
271 webm 2560x1440 1440p 6001k , vp9, 24fps, video only, 101.40MiB
264 mp4 2560x1440 1440p 6301k , avc1.640032, 24fps, video only, 125.46MiB
313 webm 3840x2160 2160p 16776k , vp9, 24fps, video only, 305.46MiB
17 3gp 176x144 small , mp4v.20.3, mp4a.40.2@ 24k
36 3gp 320x180 small , mp4v.20.3, mp4a.40.2
43 webm 640x360 medium , vp8.0, vorbis@128k
18 mp4 640x360 medium , avc1.42001E, mp4a.40.2@ 96k
22 mp4 1280x720 hd720 , avc1.64001F, mp4a.40.2@192k (best)

4Kの動画と音声を別々にダウンロード。両方ともwebmなら、音声と動画のファイル名を変えておきましょう。

PS F:\YouTubeDownload> .\youtube-dl.exe -f 313 https://www.youtube.com/watch?v=Zziq8qY6wko
[youtube] Zziq8qY6wko: Downloading webpage
[youtube] Zziq8qY6wko: Downloading video info webpage
[youtube] Zziq8qY6wko: Extracting video information
[download] Destination: Starving - Hailee Steinfeld, Grey ft. Zedd (Boyce Avenue ft. Megan Davies cover) on Spotify & Apple-Zziq8qY6wko.webm
[download] 100% of 305.46MiB in 00:16
PS F:\YouTubeDownload> .\youtube-dl.exe -f 251 https://www.youtube.com/watch?v=Zziq8qY6wko
[youtube] Zziq8qY6wko: Downloading webpage
[youtube] Zziq8qY6wko: Downloading video info webpage
[youtube] Zziq8qY6wko: Extracting video information
[download] Starving - Hailee Steinfeld, Grey ft. Zedd (Boyce Avenue ft. Megan Davies cover) on Spotify & Apple-Zziq8qY6wko.webm has already been downloaded
[download] 100% of 305.46MiB

ffmpegを使って動画と音声をMerge。

PS F:\YouTubeDownload> ffmpeg.exe -i '.\Starving - Hailee Steinfeld, Grey ft. Zedd (Boyce Avenue ft. Megan Davies cover)
 on Spotify & Apple-Zziq8qY6wko-video.webm' -i '.\Starving - Hailee Steinfeld, Grey ft. Zedd (Boyce Avenue ft. Megan Dav
ies cover) on Spotify & Apple-Zziq8qY6wko.webm' -c:v copy -c:a copy StarvingByHaileeSteinfeldCoverByBoyceAvenue.mkv
ffmpeg version N-59433-g4aa9c91 Copyright (c) 2000-2013 the FFmpeg developers
 built on Dec 29 2013 22:07:20 with gcc 4.8.2 (GCC)
 configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-avisynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libcaca --enable-libfreetype --enable-libgsm --enable-libilbc --enable-libmodplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libschroedinger --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libx264 --enable-libxavs --enable-libxvid --enable-zlib
 libavutil 52. 59.100 / 52. 59.100
 libavcodec 55. 47.100 / 55. 47.100
 libavformat 55. 22.100 / 55. 22.100
 libavdevice 55. 5.102 / 55. 5.102
 libavfilter 4. 0.103 / 4. 0.103
 libswscale 2. 5.101 / 2. 5.101
 libswresample 0. 17.104 / 0. 17.104
 libpostproc 52. 3.100 / 52. 3.100
Input #0, matroska,webm, from '.\Starving - Hailee Steinfeld, Grey ft. Zedd (Boyce Avenue ft. Megan Davies cover) on Spotify & Apple-Zziq8qY6wko-video.webm':
 Duration: 00:03:07.35, start: 0.000000, bitrate: 13676 kb/s
 Stream #0:0(eng): Video: vp9, yuv420p, 3840x2160, SAR 1:1 DAR 16:9, 23.98 fps, 23.98 tbr, 1k tbn, 1k tbc (default)
Input #1, matroska,webm, from '.\Starving - Hailee Steinfeld, Grey ft. Zedd (Boyce Avenue ft. Megan Davies cover) on Spotify & Apple-Zziq8qY6wko.webm':
 Duration: 00:03:07.40, start: 0.000000, bitrate: 139 kb/s
 Stream #1:0(eng): Audio: opus, 48000 Hz, stereo, s16 (default)
File 'StarvingByHaileeSteinfeldCoverByBoyceAvenue.mkv' already exists. Overwrite ? [y/N] y
Output #0, matroska, to 'StarvingByHaileeSteinfeldCoverByBoyceAvenue.mkv':
 Metadata:
 encoder : Lavf55.22.100
 Stream #0:0(eng): Video: vp9 (VP90 / 0x30395056), yuv420p, 3840x2160 [SAR 1:1 DAR 16:9], q=2-31, 23.98 fps, 1k tbn, 1k tbc (default)
 Stream #0:1(eng): Audio: opus ([255][255][255][255] / 0xFFFFFFFF), 48000 Hz, stereo (default)
Stream mapping:
 Stream #0:0 -> #0:0 (copy)
 Stream #1:0 -> #0:1 (copy)
Press [q] to stop, [?] for help
frame= 4880 fps=4519 q=-1.0 Lsize= 315998kB time=00:03:07.40 bitrate=13813.4kbits/s
video:312756kB audio:3137kB subtitle:0 global headers:0kB muxing overhead 0.033309%

On this day..