- Let the video buffer completely.
- Close the window so that it stops sucking 100% CPU
- Go to the .mozilla/firefox/
- $ ls -lhS
- The entries on the top are the largest, one of them will be the video. Try running them with mplayer.
- Copy and rename if the video is worth saving on disk.
This works with youtube, but I think it should work with any website that allows videos to be buffered.