This post to the Canon EF 28-135 IS group on Flickr is incredibly helpful:
- Type about:config in Firefox 3′s address bar and press Return. The configuration settings will appear.
- In the Filter field, type gfx. The list of settings will shorten to show just those related to graphics, ie gfx.
- If the Value for gfx.color_management.enabled is False, double-click anywhere on that line to toggle the setting to True.
- 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.
(thread here: Color Management PSA: Firefox 3 is now Color Managed.)
Update: In case you missed it, here’s the color profile test page: