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

Thanks for the reply

Youtube works....it's every other site I try to use a video player on that wont work.
Daily motion/Facebook/vimeo etc soundcloud also, play a track and try to move ahead and it just restarts the whole track.

it happens on safari AND firefox.

cheers

Tried re-installing flash, but as I thought made no difference.

I would re-install firefox if the problem was just on that browser, but it's on safari too.

Chrome is not an option for me....I hate it.

Update:

Called apple care and we deleted some folders under my library folder....now I can't boot my laptop passed to apple log boot screen!

Going to try a reinstall later and see what happens then!