Chances are this will be most relevant to people with screens that are capable of displaying an extended range of colours rather than everybody but here goes.
When browsing image thumbnails on MC it's very noticeable on my screen that the colours are muddy and washed out which makes image streams and portfolios in particular look drab and lacking in vibrancy. This happens because the site-created thumbnails do not contain a colour profile tag, even if the uploaded image does. In this scenario most [all?] browsers will simply pass the colour values contained in the image file directly to the screen for display without any translation. This probably doesn't work out too badly unless, as I said, your screen has extended colour capabilities like mine in which case it looks really pants!
This is also a problem if people upload images without a colour profile tag - the same logic applies. Hopefully most photographers are clued up enough to include the colour profile tag when they create their JPGs though...
Anyway the point of this post is that if you suffer the symptoms I've described then Firefox has a simple workaround. Just type 'about:config' in the address bar then search for the parameter 'gfx.color_management.mode' and change its value to '1'. Once you've restarted Firefox it will assume that all untagged image files are sRGB and the colour values will then be translated to your screen's colour profile. Unless some numpty uploads image files which haven't been outputted as sRGB then this will work perfectly!