Here’s how to get your firefox running on a 64 bit Linux machine to work with 32 bit plugins. I’ve tried this with gentoo, it should work with ubuntu or kubuntu or any other distro of your choice.

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.