After working on it the past month, the necessary bits have come together for supporting AV1 VA-API playback within Firefox on Linux. See the Mozilla.org BugZilla for tracking the progress on the effort. The latest AV1 activity in general for Mozilla can be tracked via hg.mozilla.org .
This follows FFmpeg already supporting AV1 VA-API . Granted, in order to enjoy GPU-accelerated AV1 with VA-API you need to be using the latest-generation Intel graphics or AMD Radeon RX 6000 "RDNA2" GPUs paired with Mesa 21.3 or newer with AMD having squared away their support at the end of summer .
The NVIDIA GeForce RTX 30 series does support AV1 too but there is exposed via NVDEC and VDPAU while there is an experimental translation layer for VA-API to NVDEC but that doesn't yet support AV1.