Firefox

• Install media driver following Build media driver from source code
• Install firefox if you don't have.
  Hardware accelerated video decoding is available on Intel via VA-API, it's enabled by default since firefox-101.0.1-4.
  For test purpose, install h264ify extension which makes YouTube stream H.264 videos instead of VP8/VP9 videos.
  Then play video in YouTube to check if hardware decoding worked.
  
• Install intel-gpu-tools which can help to check if hardware decoding is enabled.

$ sudo apt install intel-gpu-tools
$ intel_gpu_top

Following is an example, if have workloads in Video engine, then hardware decoding works.

Note: If hardware decoding doesn't work, you can try to set media.hardware-video-decoding.force-enable = true in about:config.