I got this idea from Linux Journal. After you have watched a video online, run this script to save a local copy in the same place this script is run.

Some pre-reqs: I assume Firefox is your browser in both Linux and OS X, and I assume you have mplayer installed in Linux and VLC in OS X.

Here’s the Linux version:

#! /bin/bash

clear

echo "Video Saver Script Foo by geezer"

echo
PID=`ps xfa | grep firefox | awk '/firefox/ { print $1 }' | head -1`
VIDEO=`ls -lU /proc/$PID/fd | grep Flash | awk '{ print $10 }' | tail -1`

DELETED=`ls -lU /proc/$PID/fd | grep Flash | awk '{ print $11 }' | tail -1`
if [ "$VIDEO" == "" ]; then

	echo "No video found!"

	exit

fi
if [ "$DELETED" == "(deleted)" ]; then

	echo "Video is no longer in cache! Please replay."

	exit;

fi
if [ "$1" == "-d" ]; then

	echo $PID

	echo $VIDEO

fi
echo -n "Name your video file: "

read NAME
cp $VIDEO $NAME.flv
echo
echo -n "Do you want to play the video now?  "

read ANSWER
case $ANSWER in

	y | y)

		mplayer $NAME.flv &> /dev/null &

		echo;;

	N | n)

		exit;;

	    *)

		exit;;

esac

And here's the OS X version:

#! /bin/bash

clear

echo "Video Saver Script Foo by geezer"

echo
PID=`ps xa | grep firefox | awk '/firefox/ { print $1 }' | head -1`
VIDEO=`lsof -p $PID | grep FlashTmp | awk '{ print $9 }' | tail -1`
if [ "$VIDEO" == "" ]; then

	echo "No video found!"

	exit

fi
if [ "$1" == "-d" ]; then

	echo $PID

	echo $VIDEO

fi
echo -n "Name your video file: "

read NAME
cp $VIDEO $NAME.flv
echo
echo -n "Do you want to play the video now?  "

read ANSWER
case $ANSWER in

	Y | y)

                if [ -f "/Applications/VLC.app/Contents/MacOS/VLC" ]; then

                   open -a VLC $NAME.flv &> /dev/null &

                else

                   echo "VLC is not found."

                   exit

                fi

		echo;;

	N | n)

		exit;;

	    *)

		exit;;

esac