I recently came across an article by Datacolor (the company who make the Spyder range of monitor / printer calibrators) which explains how to enable ICC profiles in Firefox 3. It is a very simple boolean (true/false) property which you need to change in the ‘hidden’ Firefox configuration.
In Firefox enter about:config into the address bar and press enter. A warning message will show up saying “This might void your warranty!” ignore this and click “I’ll be careful, I promise” (don’t worry you don’t have warranty anyway ;-). Next, in the filter bar type in gfx.color_management.enabled. There should now only be one option in the list of configuration options, and it should be set to false by default. Simply double click this option to change it to true (it should now be bold, indicating it is no longer the default value). Restart Firefox and you should be up and running with ICC profiles.
Check out the original post by Datacolor for more information and for two images which will indicate if your browser supports ICC profiles.