Very simple indeed. Go to this link
. Scroll down the page and select the 64bit Linux player. Download the tarball and then unpack it. It will contain a file: libflashplayer.so
. Bung that file in the ~/.mozilla/plugins folder. Restart FFx and you're good to go.
Oh, first you should remove any earlier versions of libflashplayer.so you installed earlier if there are any.