Is the stuttering throughout the video or a specific times of the video track? Also is Radeon driver the driver that is suppose to be for your hardware not downloaded drivers but the manufacture specific driver set.

Stutter seems random and advancing the video 1 frame at a time shows no missing or skipped frames. Like I said, in other players the playback doesn't stutter. Radeon drivers are from the Radeon website, not from Windows update or 3rd party sites.

So what kinda computer is it that you using? Just to see what other info others can find about it to know what drivers it loads.

CPU: Ryzen 7 5800X GPU: Radeon 6800 XT RAM: DDR4 32GB MOBO: B450 Tomahawk Max w/ latest BIOS (7C02v3F) OS: Windows 10 Home 22H2

Tried disabling all addons one at a time, used safe mode, refreshed the browser; none of these completely fixed the stutter though it did seem to lower the chance of it happening. Kind of makes it seem like the video player Firefox uses has such a low priority that even the slightest interruption makes video playback freak out. Move mouse cursor over the video, stutter. Subtitles change, stutter.

EDIT: Whenever I restart Firefox to try new settings, it takes anywhere from 5 to over 10 seconds before Firefox actually closes. Sure the window disappears when I click the X but literally nothing can be used during this shutoff time. I close Firefox and click Shutdown to turn off my PC but NOTHING happens until Firefox stops holding my PC as a hostage. But none of this is new, sometimes Firefox is the best browser, other times Edge is, occasionally Vivaldi is. I never stick to just 1 browser because of random bugs like this.

I should mention that setting layers.gpu-process.enabled to false fixes the super long FF shutdown time but then FF will occasionally crash when dragging a tab from the main window to another monitor.

I've seen similar problems, whenever I play both web video or local files. So not just http:// but also file://

Particularly whenever the CPU utilization is high enough >90%. This is not Firefox's CPU utilization, though as seen in the screenshots, CPU utilization when playing a video goes from 1% to 18 to 20%. The load roughly translate to 100% utilization on 3 threads, but spread out to 5 threads.

No such problems with standalone players or other browsers (Edge).