[mrguyorama]

I also had a 2015 MBP and watched 4k video on it basically every day. I have no idea what you experienced.

[fooker]

That's exactly how software bugs work, weird setup specific issues that are expensive to triage and fix.

But if you are not convinced see how many others have this issue by googling "site:reddit.com firefox stuttering on 4k video on mac"