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.