UPDATE: a better plugin for flash downloading has been released. Read more about it here.
As the recently released Ubuntu stable version Hardy Heron 8.04 comes with Firefox 3 (beta), my previous used plugins to download videos from Youtube aren’t currently supported, as they are mainly written for Firefox 2.x.
Therefore I had to use the Flash Video Resources downloader to be able to download those videos, which works flawlessly.
Clicking this blue symbol in the bottom right corner enables you to download the video you currently loaded via youtube:
A new page will popup after clicking the symbol, which is self explaining. Just hit the “Get Video” button, right click on the appearing “Get Flv” Field and hit “Save as” in the menu popup of firefox.
It is important, that you rename the video according to the desired name, following the type .flv.
That’s it.

Posted by Rock 
Posted by Rock