The Firefox add-on Video Downloadhelper should be able to extract most video
clips from websites.

I also use downloadhelper a lot.Site:
https://addons.mozilla.org/en-US/firefox/addon/3006 There is also youtube-dl
which can be installed by the command sudo aptitude install youtube-dl and when
using it open the Terminal, type youtube-dl hit space then copy and paste the
URL of the site of the online movie and it downloads it to the Home folder.
But Downloadhelper is a lot better. It also creates a Dwhelper folder in the
Home folder where it downloads the movies, usually files with .flv extension,
which can be played with Mplayer or, better, VLC Player.
Both players are in the repositoriessudo aptitude install vlcsudo aptitude
install mplayer

For YouTube and some other video sites, you can use the KeepVid service
at http://keepvid.com/

I addition to the previous comments about Firefox add-ons, you can often find
the file in your tmp folder in your root file system. It will be a large file
with an alphanumeric name, often beginning with capital F, and it will grow as
the video is streamed, but it will disappear after you close the viewing window.
This does not always work but most often it does. I just drag the completed file
to my desktop and re-name it.

I loaded the fire fox download helper and this went okay. I went on You tube
and downloaded a video and this went okay. When I tried to play the video I got
this message: "Totem Movie Player requires additional plugins... Flash demuxer."
Does anyone know about this?

For .avi and .mpeg files it is easy to install the plugins. but for .flv files
it is better to use either vlc player or mplayerI installed both and I prefer
VLC player
Both are in the repositories. they can be installed with sudo aptitude install
vlcsudo aptitude install mplayerAfter installing these you can choose whatever
movie player you want to use for playing a movie. I prefer VLC for .avi, .wmw
and .mpeg files, also.

I agree with the previous comment to try VLC for playing flash videos. However,
they play in Mplayer or Totem for me which tells me that you don't have all of
the multimedia codecs installed. Have you added the restricted extras?

here is an efficient method :
you go to your home directory, you enter into "tmp" folder, you will find the
video there :d

I have

~.mozilla/firefox/xxxxxxx9.default/cache

but I can't locate any tmp directory other than ~var/tmp
which contains a kdecache directory for each user.

What would this refer to?

The tmp directory is in the root tree. Click the up arrow in the file manager to
go higher than /home. Look for /tmp.

However, this does not always works. Some files for copyright reasons do not
display there.

Right, I remember now having seen it there. But how come
the locate command didn't find it? Or, maybe it did --
at the beginning of its search -- but the list of files
containing the string "tmp" was so long that it consumed
the whole buffer. I couldn't scroll to the top of the list
there were so many files. Then, locate --help did not
explain how to use the command to locate directories only,
nor how to omit all the substrings. Maybe I should have
piped it through grep or something...