I'm still on a NVIDIA GeForce GTX 760, but I have a 4k monitor (X11, desktop). Perfectly smooth in Chromium browsers, but YouTube and Street View are basically unusable in Firefox. Before when I had a Full HD monitor it was usable in Firefox, though even back then Chromium based browsers where a bit smoother at those tasks. And yes, I've set all the flags in about:config that you can find on that issue. No change.

But I need to upgrade my PC. When I do I'll try Firefox again. Would be awesome if it would work for me again.