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

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.