For the past few years I've been using the youtube-dl Python script to download youtube videos on my 64-bit linux box (thanks for dropping the ball on a 64-bit flash player Adobe ... thankfully GNU gnash has become more stable recently).
Except that this version seems to make Firefox massively more unstable.
It's not great, but I've had fewer problems with it than with a 32-bit build with nspluginwrapper. I use flashblock, so it's not really a problem for me.