I came across this post on another forum:

#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.
Not being anything approaching technically minded, I thought I'd post it here as it might have some relevance to us in terms of colour matching - but I would be grateful if someone more technical than myself could explain what the advantages are.

As a side note, the brown frame around my avatar is invisible on my computer at home (running Firefox) but is a slightly different shade from the brown background on my computer at work (running ie 7) - I simply cannot understand why this should be the case - but gave up worrying about it a long time ago!