I'm not getting HDR output from the Firefox browser on macOS (but I am on Chrome and Safari). 'Color' field in stats for nerds shows bt709 in firefox (i.e. not HDR) and smpte2084 (PQ) / bt2020 in Chrome/Safari (i.e. HDR).

Also side by side the Firefox picture is flatter and less bright.

You can always build and sign something with your own cert (if you have a kext cert in this case)