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

To enable color management 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.).