I have tried to install the Beta version of Flash using the Firefox plugin Flash-Aid with no luck. The Beta version right now is the 11.2 and I wanted to test it how it changed in performance wise but I seen to find it difficult to install it.
First of all, uninstall any Flash version on your system. Download the suitable (32bit or 64bit) installer here: http://labs.adobe.com/downloads/flashplayer11-2.html
Extract the archive and put libflashplayer.so file in /usr/lib/mozilla/plugins for system wide:
For user only: