clear the cache and cookies. disable your extensions and give it a try. sometimes it happens when the internet connection is not stable. check the network connection.

It does not go, I had to reinstall Firefox 56.0, I have similar problem on Opera latest version, Firefox 76.0b4 Developer and Microsoft Edge, using these images tremble or block but the video contina because I hear the audio, I have already tried the suggested options. The PC is an 8 core intel xeon e5-2670 2.60ghz Mobo Intel DX-79SI sk2011, with 16gb DDR3 ECC 1866, Video card: AMD HD 5970 X2 GPU 2GB DDR5 drivers updated, OS Windows 7 x64 sp1, updated. This happens on other desktop PCs that I have, same software and OS, different platforms, cpu intel i7-990x and i7-980x 6 cores.

Have you tried something like the Firefox add-on Enhancer for YouTube where you can set "Automatically play videos at my preferred quality" in the settings. Enhancer for Youtube