Summary
When playing Flash video in Firefox, the screen saver is not disabled and so you are required to move your mouse every few minutes to prevent the screen saver from turning on. This is highly annoying. This guide provides a method to automatically disable the screen saver whenever Flash is being used in Firefox.

Note: This method disables the screen saver regardless of what type of Flash program is running. It doesn't discriminate between a Youtube video and a Punch the Monkey ad.

Implementation
We are going to create a daemon that detects if the Flash shared library currently has any memory mapped. If it does, we disable the screen saver. If it doesn't, and we know that we previously disabled the screen saver, we turn it back on. First, we need to make sure we have a local bin directory:

Code:
mkdir -p ~/bin
Next, save the following script as ~/bin/flash_saver.sh:

Code:
#!/bin/bash

# Cleanup any bad state we left behind if the user exited while flash was
# running
gconftool-2 -s /apps/gnome-screensaver/idle_activation_enabled --type bool true

we_turned_it_off=0

while true; do
    sleep 60
    flash_on=0

    for pid in `pgrep firefox` ; do
        if grep libflashplayer /proc/$pid/maps > /dev/null ; then
            flash_on=1
        fi
        
        ss_on=`gconftool-2 -g /apps/gnome-screensaver/idle_activation_enabled`

        if [ "$flash_on" = "1" ] && [ "$ss_on" = "true" ]; then
            gconftool-2 -s /apps/gnome-screensaver/idle_activation_enabled \
                --type bool false
            we_turned_it_off=1
        elif [ "$flash_on" = "0" ] && [ "$ss_on" = "false" ] \
                && [ "$we_turned_it_off" = "1" ]; then
            gconftool-2 -s /apps/gnome-screensaver/idle_activation_enabled \
                --type bool true
            we_turned_it_off=0
        fi

    done
done
Next, we make the script executable:

Code:
chmod +x ~/bin/flash_saver.sh
Now, we can turn it on by pressing F2 and running, "~/bin/flash_saver.sh". You can now either test it watching some Flash videos or just proceed to System->Preferences->Session and adding "flash_saver.sh" as one of your startup programs.

Testing
Watch some long Flash videos and verify that screen saver doesn't come on. Afterwards, close all tabs that have any Flash content on them and see if the screen saver now comes on as normal.