Mac EL Capitan Firefox youtube video flickering when progress bar is shown

I am running OS X 10.11.6 and I have a strange problem with video. When I watch youtube video in FireFox (50.1.0, updated often), I see video flickering (fast blink) whenever the progress bar is shown. That includes when I move my mouse or kept my mouse pointer on the progress bar.

It seems that whenever there is an image overlay, the video will flicker. This does not happen in Safari but happens when I watch Amazon video in Safari. This problem has been bothering me for months. I cannot figure out how to resolve it. Please help!

Edit: I'm using the native HTML5 codec. Flash is disabled.

Here is an example of video causing problem: https://www.youtube.com/watch?v=coDff5RLIas

However, I noticed that not all video flicker as bad. Here is an example that seems to only flicker in full screen: https://www.youtube.com/watch?v=coDff5RLIas

It seems like Hardware Acceleration solves the flicker problem. This option is turned on by default, so it is a bit puzzling why it was off for Firefox.

To explain what it does : GPU-accelerated computing offloads compute-intensive portions of the application to the GPU, while the remainder of the code still runs on the CPU. While the CPU is designed for general-purpose operations and consists only of a few cores optimized for sequential serial processing, a GPU has a massively parallel architecture consisting of hundreds if not thousands of smaller, specialized cores, designed explicitly for graphical operations.

That's why using GPU operations, the display of video frames is accelerated. As each video frame is timestamped, if its display is not fast enough and takes too long, the video playback may need to skip a few frames, which will cause a flicker.

to enable Hardware Acceleration in Firefox, use the menu Tools > Options > Advanced > General > Browsing, then check "Use hardware acceleration when available".

Whether Hardware Acceleration works or not is depended on the video card and its driver. Some unfortunate users are forced to turn it off.