No browser on Linux supports any other backend for video acceleration except VAAPI, as far as I know. AMD and Intel use VAAPI, while Nvidia uses VDPAU, which is not supported anywhere. This single fact means that with Nvidia graphics cards on Linux, there isn't even such a simple and important feature for users as video decoding acceleration in the browser. Every silly YouTube video will use CPU (not iGPU, but CPU) to decode video, consuming resources and power.
Yes, there are translation layers[1] which you have to know about and understand how to install correctly, which partially solve the problem by translating from VAAPI to NVDEC, but this is certainly not for the average user.
Hopefully, in the future browsers will add support for the new Vulkan Video standard, but for now, unfortunately, one has to hardcode the browser launch parameters in order to use the integrated graphics chip's driver (custom XDG-application file for AMD APU in my case: ~/.local/share/applications/Firefox-amdgpu.desktop): `Exec=env LIBVA_DRIVER_NAME=radeonsi DRI_PRIME=0 MOZ_ENABLE_WAYLAND=1 __NV_PRIME_RENDER_OFFLOAD=0 __GLX_VENDOR_LIBRARY_NAME=radeons i /usr/bin/firefox-beta %u`.