If you're using Firefox in OS X and watch a video on YouTube, is the video saved in a temporary folder somewhere? Where can it be found?
On my system, it doesn't look like any
but these files are deleted when the youtube pane is closed.
To find out where files are being stored on your system, I recommend fseventer. As I have recommended it twice in 5 minutes, it might be appropiate to mantion that I'm not affiliated with fernlightning ;-)
If you want to save youtube videos, try out DownloadHelper.