For (almost) any flash video that you are watching, look in the /tmp folder for a strangely named file. It is generally the last one (use ls -ltr). Once the video completes downloading, do not navigate away from the page (it will remove the tmp file). Instead copy that file in /tmp/ to somewhere else with another name and run it with mplayer. I use this method to keep flash videos for off-network play.

Note, the copy is not necessary to play it – you can play it directly, just don’t rely on the Ubuntu tab completion as it expects certain extensions. The mplayer -identify will give you hints as to how to name the extension.