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.