You can see if you fully support the HTML5 player by going to https://www.youtube.com/html5 as all checks should be blue and not red.

For Linux users make sure you have FFmpeg.

When I enable the WebM options in about:config all the checks are blue on the site you linked. FFmpeg is installed (binary under /usr/bin/ffmpeg, it seems to be working). Video is still jittery. I'm not sure if firefox is using ffmpegs shared libraries or running it as a subprocess. While I'm playing the video ffmpeg is not running.

Try turning off Hardware Acceleration Options ---> Performance Untick Use Recommended Performance Settings --> Untick Use Hardware Acceleration

You may loose Open GL but Stuttering should stop.

Please let us know if this solved your issue or if need further assistance.

With WebM enabled and Hardware Acceleration disabled it's still stuttering. CPU usage with and without Hardware Acceleration seems about the same, and it's high. My CPU is relatively weak but my GPU is pretty strong. With chromium the CPU usage is lower and the video plays back smooth. I suspect that Firefox is not using my GPU with and without Hardware Acceleration.

For cpu/ram usage last comment.

Check to see if 57.01 fix took : go to the Firefox 3 Bar Menu --> Options --> Privacy & Security panel and under Permissions check the setting to Prevent Accessibility Services from accessing your browser. Restart Firefox

No all in all is possible could be issue but check, it is also for cpu/ram : You could try this please : Go the 3 Bar Menu then Options --> General --> Performance and untick everything. change the recommended size lower then see how it runs. Note: 1 = No Multiprocessor = slow again. Try 2 Restart Firefox after making these changes please.

Please let us know if this solved your issue or if need further assistance

"Prevent Accessibility Services from accessing your browser" doesn't seem to have any effect. With hardware accel disabled and 2 processes instead of 1(default) it seems to be a tiny bit better but that might be my imagination. There are always firefox with about 100% core usage and "Web Content" with 100%-200% core usage running. There's definitely more CPU power available which it isn't using.

Again, it might be that it is playing a perfectly fine 30fps video. But on chromium the same video feels way smoother. I downloaded the YouTube video using youtube-dl and playing it locally feels as smooth as with chromium. Maybe YouTube is providing different qualities/formats of the video and Firefox is choosing a 30fps version.