Color management does not work in Firefox, that is, it ignores the color profile of the srgb monitor and the colors are distorted.

Make sure that the about:config is set to:

gfx.color_management.enablev4 - true	
gfx.color_management.force_srgb - true	
gfx.color_management.mode - 1	
gfx.color_management.native_srgb - true

Image for testing. The gradient should be smooth (compare with Chrome, maybe it’s in your monitor, software, etc.).