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.

They could, probably. My guess is, that it’s either a limitation of resources, the issue of licensing fees or Google‘s significant financial influence on Mozilla forcing them to make a worse browser than they potentially could. Similar to how Firefox does not support HDR (although, to my knowledge, there’s no licensing involved there).

The biggest problem most people have with Mozilla is said influence by Google, making them not truly independent.