(In reply to esmorun from comment #3)
I believe the problem is that color profiles are not working on OSX.
On the below site, I only see a red blob in Firefox, suggesting that it does not display the DCI P3 colour space correctly. In Safari I see a logo and brighter red color on the right side.
I had the same problem on my Windows machine and fixed it by specifying the path to my ICC profile (thats another bug I guess):
gfx.color_management.display_profile = "C:\Windows\System32\spool\drivers\color\C27HG7xQQ.icm
I tried the same trick on OSX, but the settings have no effect:
gfx.color_management.mode = /Library/ColorSync/Profiles/Displays/Color LCD-37D8832A-2D66-02CA-B9F7-8F30A301B230.icc
gfx.color_management.enablev4 = true
gfx.color_management.mode = 1
Testing was done with Firefox 89.0.1
Thanks @esmorun, this helped solved my issue (running Firefox 102.4.0esr on a Mid 2011 iMac, macOS 10.13.6).
Just like you I couldn't see the DCI P3 logo on Firefox, but could on Safari. The following settings tweaks in about:config on Firefox fixed it so that Firefox displays the P3 image correctly:
gfx.color_management.mode = /System/Library/ColorSync/Profiles/Display P3.icc
gfx.color_management.native_srgb = false
Note, the other gfx.color... settings didn't seem to make a difference, these are the important ones to adjust!