Just created an account to share my findings. I'm currently running version 130.0.
Go to the web page "about:config" in your Firefox. In the search bar at the top, type "hevc". It should find "media.wmf.hevc.enabled". @wutongtaiwan
If enabled, you should have a "1" at the end of that line. If you have a "0", "hevc" is disabled. If you have it on "0", simply double-click the "0", then type in "1" and then press Enter.
@itskarenI would REALLY like to see HEVC for non-DRM content. Use-case: Ubiquiti now supports HEVC for their video camera software. If I enable HEVC for my cameras, now I can't use firefox anymore to watch my cameras. I don't want to have to switch browsers just for that. Start a GoFundMe or whatever!