#1 Type about:config in Firefox 3's address bar and press Return. The configuration settings will appear.
#2 In the Filter field, type gfx. The list of settings will shorten to show just those related to graphics, ie gfx.
#3 If the Value for gfx.color_management.enabled is False, double-click anywhere on that line to toggle the setting to True.
#4 Quit and relaunch Firefox 3 and you're in business. You can confirm that colour management is working by viewing the photos on this page. If all four quadrants of the first photo are a seamless match, then colour management in your copy of Firefox is up and running.