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Announcing the Ladybird Browser Initiative
(ladybird.org)
This is a most excellent place for technology news and articles.
HEVC is almost entirely down the the licensing. This section of the wikipedia page details it pretty well.
The tl;dr is that the LA group wanted to hike the fees significantly, and that combined with a fear of locking in led to the mozilla group not to support HEVC.
And it's annoying at times. Some of my security cameras are HEVC only at full resolution, which means I cannot view them in Firefox.