I looked at this bugzilla issue and some other pages and changed the flags in about:config. When I look at this page, which shows P3 vs non-P3 colors, I don’t see a difference, but do in Safari and Chrome.
gfx.color_management.display_profile = /System/Library/ColorSync/Profiles/Display P3.icc
gfx.color_management.mode = 1
gfx.color_management.native_srgb = false
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