Safari/Firefox: Can't skip ahead on videos etc

Lately I find that when watching a video on any site other than youtube I can't skip ahead in the video without it stopping and going back to the start of the video.

Happens on Soundcloud too.

I have tried disabling and extensions I have on both browsers, but no joy.
This is only happening on one of my 2 macbook pros (both on 10.8.3).

Is there any solutions you could suggest?

It happens exactly the same on both browsers and I'm lost as to what could be the reason or what to do.

Thanks