Set gfx.color_management.mode to 1 to enable colormanagement. The parameter changes to bold text which indicates that it is now user-defined.

Below are all parameters which can be altered:

gfx.color_management.display_profile The Full Path to you monitor’s color profile. For example: C:\colorprofiles\HP Z27x\HP Z27x sRGB D65 ICC Profile.icm

gfx.color_management.enablev4 Set to true (doubleclick the entry) to enable colormanagement v4 (Recommended).

gfx.color_management.mode 0 – Disable color management. 1 – Enable color management for all rendered graphics. (Recommended) 2 – Enable color management only for tagged images only. (Default)

gfx.color_management.rendering_intent -1 Honor the rendering intent specified in the image file. 0 Perceptual (Default) (Recomended) 1 Relative colorimetric 2 Saturation 3 Absolute colorimetric

To finaly enable color management you have to restart firefox.