Internet speed check, from 4 different sites averages at just under 11 Mb/sec, with the occasional blip up to 16Mb/sec. Not exactly slow. As a matter of interest, I have realised that when I downloaded the Flash Player thingy for Firefox there was another version for IE. So I downloaded that and videos now work in IE, Chrome and I also tried Safari which also works. So the only one which doesn't is FF, FFS!
Firewall checked - not that I have ever done anything to it.
I'm not tearing my hair but it is a bit baffling. No doubt there is a very simple answer.