Youtube not working correctly using Firefox

Laptop Lenovo Ideapad 300 Windows 10.
Youtube works perfectly using Internet Explorer. But using Firefox youtube will not play unless I hit pause
and then play. It's weird but I don't think it's on my end. I started with the basics in disabling all FF addons
to no avail. I even booted an external USB cloned hard drive I made last week or so when FF youtube was working.
But even this cloned drive does the same thing. Open youtube, choose a clip, it sits there looking stupid for a few
seconds and then says to reset and try again at which time I hit pause/play and it goes merrily on its way.
Besides using a cloned drive I downloaded a couple older versions of FF also to no avail. I'm using FF 40.0.2.
Any suggestions would be appreciated.
Thanks.

Sorry for the typo but I was running version 48.0.2 (not 40.0.2). Now I have 49.0b10 with same results.
If you followed the op, isn't it weird my cloned system from last week did work but is doing same thing now?
Click pause, then play and away it goes as it should. Am I being picky? Well, yes. I expect it to work correctly.
Thanks for the responses.

I had the latest version of Flash until it was upgraded again about an hour ago.
As stated above I disabled all FF add-ons which are plugins and extensions.
This is so weird and I am not a novice. With a computer and networking engineering background I must say
I'm stumped. I booted a cloned drive that worked when I did that but has same prob now. Funny.
Not life or death but I'm not one to leave things dangling. I will get to the bottom eventually and will advise.
Just for grins I may put an older drive with W7 just to appease my puny mind.
Thanks for your input.

Are you saying that you meet the problem that YouTube not working on Firefox again? Maybe you can turn off AdBlock if you have installed it on your browser. And how about the HTML5 player and the hardware acceleration? I don't know if these are useful so you can check more on Effortless Methods for Fixing YouTube Videos Won’t Play Issue or YouTube does not load properly in Firefox, but does in other browsers. | Firefox Support Forum | Mozilla Support