Whilst Firefox may support hardware video decoding, Mesa since March 2022 disables patent encumbered codecs by default[1], and distributions such as Fedora and OpenSuse do not explicitly enable these patent encumbered codecs to avoid possible legal problems. Even Gentoo (built from source code by the user) requires the user to explicitly enable a USE flag (proprietary-codes) to use patent encumbered codecs.[2]

The thought process is that AMD, NVIDIA, Intel and the likes are not providing a patent license with their hardware.[3] They are instead just supplying part of an overall system that together with operating system kernel, display manager software, video player software, etc allows the decoding and encoding of patent encumbered video files. Open source software projects and distributions are concerned they'd be found to be infringing patents by enabling a complete solution out-of-the-box. Hence they put some hurdles in place so that a user has to go out of their way to separately piece together the various parts to form a complete system capable of encoding and decoding patent encumbered codecs.

edit: To clarify, if your Intel or AMD GPU/APU supports a patent-free codec such as AV1 (most GPUs/APUs available for sale?), Firefox on a standard Linux distribution will use hardware video decoding out of the box by default for the patent-free codec. The issue is really one of whether you're sourcing content from a provider that uses a good choice of codec like AV1. The good news is that patent trolls are doing a good job of pushing laggard content providers down this path.[4]

[1] https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15...

[2] https://github.com/gentoo/gentoo/commit/1265a159743d7f07185a...

[3] https://lists.fedoraproject.org/archives/list/[email protected]...

[4] https://news.ycombinator.com/item?id=38249527