I assume you're using the Wayland backend? Because on X11 we've been supporting this for years I AFAIK. On Wayland it only landed in bug 1645528 - shipping in 86 :)
Bug 1687909 Comment 2 Edit History
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I assume you're using the Wayland backend? Because on X11 we've been supporting this for years I AFAIK. On Wayland it only landed in bug 1645528 - shipping in 86 :) P.S.: or on X11 using software rendering - Firefox only has vsync and thus frame rate detection on X11 when using hardware rendering. You can check https://wiki.mozilla.org/Platform/GFX/WebRender_Where to see where it's enabled by default - it will get more over the next couple of releases!