I just tested a video on YouTube. My video played with everything as it is supposed to be. I know Firefox 3 is much better with YouTube that Firefox 2 was. Sometimes YouTube has bad videos, so bear that in mind. If you do get a blank screen, refreshing the page or even going to another page and returning can be enough to reset whatever may have gone wrong. There has always been a slight YouTube/Firefox/Linux issue, but it is better than it was. Of course, this is the fault of Adobe, not Linux Mint.