youtube videos no longer work

[Yossarian]

This PC runs Mint 9 Isadora and has recently stopped playing internet videos - youtube shows an empty black box, other sites eg BBC news show nothing at all. It used to work fine but I don't remember what's changed since it did last work. As far as I can see I have the latest flash player installed 11.2.202.233 browser is firefox 12 for linux mint 1.

Any ideas?