For NVIDIA users who want to enable hardware acceleration for Firefox video playback, NVIDIA VA-API driver released v0.0.14 few days ago with improved compatibility and various fixes.
As you may know, Firefox in Linux does NOT support NVDEC (NVIDIA Video Decoder) to accelerate video decoding. As a workaround, user may use the free open-source nvidia-vaapi-driver
as VA-API implementation that uses NVDEC as a backend.