Flash-Aid is an amazing add-on for Firefox capable of removing any conflicting plug-ins and installing newer version including the beta one's directory from Adobe Labs as well as those from the official repositories. Using it is simple, just go the link below in Firefox and click 'Add to Firefox':
Once installed, an icon would be placed in your Firefox toolbar.
Click this icon and choose 'Wizard Mode'. Choose whichever version of Flash you want to install, want to apply any optimization tweaks etc. When you click finish, the script would run some commands in a Terminal which would require you to enter you password. Once the script finishes its job, it would notify you. You can then close the Terminal and restart Firefox to enjoy your latest Flash Player.