Yo recomiendo esto:
Código:
https://addons.mozilla.org/es/firefox/addon/download-youtube/?src=userprofile
Según la opción dice lo siguiente:
Código:
What are the download options?
Here are the formats you can download using this extension:
* MP4 360p (480x270, 480x360) - all videos are available in this format. Use it if you want to play videos on an old mobile phone (iPhone 3GS, HTC Desire, Samsung Galaxy S2) or to save space.
* MP4 720p (1280x720) - HD format, available for an increasing number of videos. Use this format if you want to play higher quality videos on a tablet, laptop or a phone with HD display (Samsung Galaxy S3, Galaxy Nexus) or almost HD display (iPhone 4/4S/5).
* MP4 1080p (1920x1080) - Full HD format, available for less videos than MP4 720p. Use this format to watch videos on a Full HD TV, a high resolution monitor, high resolution tablet (iPad 4, Nexus 10) or Full HD phone (Samsung Galaxy S4, HTC One)
* MP4 4K (up to 4096p) - HD format, available for a few videos.
* just in case you need it, you can also download the highest-quality FLV version available: FLV 360p (320x240, 400x226, 640x360) or FLV 480p (640x480, 854x480).