THIS IS STILL IN BETA MODE & FOR 64-BIT VERSIONS OF LINUX ONLY! INSTALL AT YOUR OWN RISK!
To install via PPA simply open terminal, and enter:
sudo apt-add-repository ppa:sevenmachines/flash && sudo apt-get update && sudo apt-get install flashplugin64-installer
Or you can get the more recent Flashplayer 11 RC1 from here:
http://labs.adobe.com/downloads/flashplayer11.html
but you’ll have to install it manually 
Download the 32 or 64bit version (depending on your Linux distro architecture), extract the libflashplayer.so file, and put it in -
~/.mozilla/plugins
(if the plugins directory doesn’t exist, create it… also be aware the .mozilla directory is hidden, so you’ll have to enable “Show hidden files”)
Then, close ALL instances of Firefox, and reopen it.
Lastly, in Firefox go to Tools>Add-ons>Plugins and make sure the ONLY Shockwave Flash that is enabled is version 11.0 r1
For users who don’t know how to “show hidden files” hit keys: ctrl & h at the same time.