I thought everything on a new pc would work perfectly! After installing the latest version of Firefox, I soon found youtube videos wouldn’t play. The videos would stop after a second or two. After a few searches, I found lots of other people had found the same problem. The answer appeared to be updating the flash player, which I did – but still the same problem. The solution was to remove the older version of flash player, which I suppose was installed by Firefox? Wherever it came from, it needed updating, so if anybody has the same problem and does a search & finds this – update your flash player & remove the old version of flash player.