How To Install Flash on Ubuntu AMD64
Regretably, Macromedia's Flash player for Linux won't work out of the box on AMD64. Since we have a lot of 64 bit workstations with Ubuntu at Bitscribe, we've put together these directions for getting Firefox to run in 32 bit mode for the purpose of using Flash to view the screencasts. These instructions assume that you're using Dapper, though other versions may work too.
Make sure that /etc/apt/sources.list has the universe and multiverse lines uncommented, i.e.
deb http://us.archive.ubuntu.com/ubuntu/ dapper universe deb http://us.archive.ubuntu.com/ubuntu/ dapper-backports main restricted universe multiverse
Then execute:
sudo apt-get install ia32-libs gsfonts gsfonts-x11 alsa-oss ia32-libs-gtk lib32asound2 lib32ncurses5 ia32-libs-sdl linux32
mkdir firefox32flash
cd firefox32flash
wget http://adam.blogs.bitscribe.net/downloads/firefox32-1.5.0.6_amd64.deb
sudo dpkg -i firefox32-1.5.0.6_amd64.deb
wget http://fpdownload.macromedia.com/get/flashplayer/current/install_flash_player_7_linux.tar.gz
tar xzvf install_flash_player_7_linux.tar.gz
sudo cp install_flash_player_7_linux/{libflashplayer.so,flashplayer.xpt} /usr/local/firefox32/plugins
Now Firefox can be executed in 32 bit mode by running firefox32 (i.e. at the command line, or via the Alt-F2 run dialog). If you'd prefer to use the 32 bit mode all the time, use the System -> Preferences -> Preferred Applications dialog to set a custom command firefox32. (The only downside to running it all the time is a slight performance hit for running the app non-natively.)
The 32 bit Firefox .deb was made by Kilz and the instructions based on his Ubuntu forum post.