The next major release of Firefox (3.5) has Colour Management turned on by default.
You can turn it on now (3.0.x) using this method...
Type "about:config" in the address bar of Firefox 3.
Click thru confirmation page and find: gfx.color_management.enabled
Double click that until it says "true".
Then restart Firefox 3.
Hint: Images posted for comps should be in sRGB colour space as not everyone has colour managed browsers and that is the only way to ensure your image presents well to all potential voters.