install flashplugin-nonfree and nspluginwrapper
download latest flash plugin and
# sudo cp libflashplayer.so /usr/lib/mozilla/plugins/libflashplayer.so
# sudo nspluginwrapper -i /usr/lib/mozilla/plugins/libflashplayer.so
idea comes from here.
Of course it does'nt work perfect: I can watch maybe 1 or 2 videos, and than it quits, and I have to restart firefox....
After all, this doesn't work. So what remains: 32bit firefox again.