gfx.color_management.mode = 1 only worked vaguely correctly if all of your monitors share the same color space (or you only have a single monitor). Videos also weren't color managed, and there still existed some situations where colors would still be incorrect. (see: https://cameratico.com/tools/web-browser-color-management-te...)
This change effectively sidesteps the issue by making everything render to sRGB, and telling macOS that Firefox's windows are sRGB only.
This change effectively sidesteps the issue by making everything render to sRGB, and telling macOS that Firefox's windows are sRGB only.