TubeRadio.fm - Turn YouTube into a jukebox anywhere you are

TubeRadio.fm - Listen to and watch popular and rare music videos for free with the internet's largest music catalog. TubeRadio, Youtube for Music.
Great for DJing parties.
Watch the demo video at the bottom of the page.

Remove jerkiness in You Tube videos

YouTube Auto Buffer & Auto HD & Remove Ads for Greasemonkey
Add Greasemonkey, then add this script.

Downloading embedded on-line videos

If you want to download an embedded video, here are a couple of options:

For YouTube videos, there is the easy to use bookmarklet that enables you to download the video in MP4 format:
Download YouTube Videos as MP4 Files
Or,
You can use the Orbit YouTube Downloader - more complex:
Orbit YouTube Downloader

For a variety of embedded video formats, there is the full Orbit Downloader
Orbit Downloader

All of these are free.

Sure, there are Firefox extensions that enable you to download embedded videos, but they don't always work. When they don't, try the above downloading methods.