You can add any YouTube video / audio in every format (html5, flash, mp3) with these options:
– icon (plus) next to logo on YT video detail page
– context menu: right click to background on YT video detail page
– context menu: right click to any link (on any webpages), that directed to youtube video
Next you can play in the popup your favorited videos without go to youtube.com and without download(mp3) this video. Important button is Detailed view, which open player in detail mode on tab.