I have has this problem for ages and have been cursing all and sundry. However I have found an article that has fixed it for me. It turns out that during the upgrade to Windows 10 my Power settings were changed from balanced to stable. I reset the power saver settings back to Balanced (recommended) and now Firefox is back to it's original speed. Videos play without buffereing now and no annoying waiting for script message Yeahhhhhhhhhhh