> Firefox currently supports hardware video decoding with Intel's vendor specific VA-API only on Linux, which is not supported by NVIDIA.
(emphasis added)
You further wrote:
> Firefox uses Windows Media Foundation, which is cross-vendor, on Windows. It uses MediaCodec on Android which is again cross-vendor.
And? None of those APIs are cross-platform. Vulkan Video will eventually allow developers (including Firefox developers) to write a single code path for video to cover a wide range of platforms and vendors (likely with the exception of walled gardens like Apple-land, although someone might find a way to support like via a wrapper like MoltenVk for Vulkan).