There are several plugins for Firefox that allow you to save content. It basically analyzes the source code and searches for a video file (.flv or .mp4) and downloads it directly. The flash player on the page simply plays the provided file. They could, of course, confuse the path to the video file, but it could also be reverse engineering. They can not do anything about it, because the video file must be on the user's computer at some point, or if not, the stream can also be intercepted.
eg. https://addons.mozilla.org/de/firefox/addon/6584/?src=api
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