The YouTube Plus Plus extension for Firefox is brilliant. You can watch a video while you search another one on the same page. Thanks to HTML5, you can easily create playlists and save them locally.
Here are some of the features of this add-on:
- watch and search videos without switching between windows or tabs
- create playlists and save them locally (thanks to HTML5)
- discover music and video according to your taste
You can pick up YouTube Plus Plus at the Firefox Add-ons website.