Vulkan is the modern option, the difference is not being stuck with legacy paths and using something that allows explicit sync.
Wayland is also the modern option, so I don't really worry about X11 use cases. For remote desktops, better to use something like FreeRDP anyway. X11 forwarding is much worse in every sense.
I think KDE are working on integrating FreeRDP server into Plasma for seamless usage.
Another thing to add for Firefox would be may be switching to Vulkan video from VAAPI (or at least having it as an option since ffmpeg already supports it) and using hardware acceleration for video encoding too, not just for video decoding.
