I managed to reproduce this by setting media.ffvpx.enabled to false in the about:config tab on Firefox 87(20210318103112) on Linux x86_64(Ubuntu 20.04). This issue is fixed on the latest Firefox beta 98.0b4(20220213185901) and Nightly(20220214092817) builds on the same OS; hardware decoding works as expected.
Bug 1705940 Comment 9 Edit History
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I managed to reproduce this by setting media.ffvpx.enabled to false in the about:config tab on Firefox 87(20210318103112) on Linux x86_64(Ubuntu 20.04). This issue is fixed on the latest Firefox beta 98.0b4(20220213185901) and Nightly 99.0a1(20220214092817) builds on the same OS; hardware decoding works as expected.