Download and install nspluginwrapper from http://gwenole.beauchesne.info/projects/nspluginwrapper/.

For ubuntu, there are some instructions on how to do this here (http://ubuntuforums.org/showthread.php?t=341727). On gentoo I had to

 #emerge nspluginwrapper

Download the flash installer from the website here: http://fpdownload.macromedia.com/get/flashplayer/current/install_flash_player_9_linux.tar.gz. Untar the installer:

$ tar -zxvf install_flash_player_9_linux.tar.gz

Step into the installer directory:

cd install_flash_player_9_linux

Edit the installer file:

vi flashplayer-installer #use nano or gedit if you like

Navigate to line 47 (this inside the function exit_cpu) and comment out the line that says exit 1.

# the architecture is not supported
exit_cpu () {
  echo ""
  echo "ERROR: Your architecture, \'$1\', is not supported by the"
  echo "       $PRODUCT installer."
  echo ""
  #exit 1   # this was line 47
}

Install the flash plugin, I used the default paths, you can choose the paths according to what you have on your computer (I used /home/ketan/.mozilla)

./flashplayer-installer

Now verify that the plugin actually got installed in the directory that you specified

$ ls /home/ketan/.mozilla/plugins

Use nspluginwrapper to create a 64-bit wrapper around this 32 bit plugin. (thanks to Lance for a typo update)

$ nspluginwrapper -i /home/ketan/.mozilla/plugins/libflashplayer.so

Now restart firefox, that should get you to run firefox with flash player.