Hi Ian,
There was an issue recently (a couple of weeks ago

) with a Flash Player upgrade...I'm on Firefox (W7 64) and FP upgraded then suddenly youtube, etc., wouldn't play..black screens and so on.
I did a google, sorry can't be specific, 'flash player black screen' or 'youtube won't play' or something like that and found some results with links to earlier versions..uninstalled, reinstalled previous version and that did the trick
I haven't allowed FP updates yet, just in case.
Hope this helps.
Derek
edit, just noticed that you have 64bit too, that may be the common denominator
