As Chris says, if you apply colour management to your browser, it allows you to see the range of colours contained within any profiles embedded in the image. This is ONLY true if the monitor you are using is capable of displaying the range of colours presented. This is where Clive is coming from, as most monitors can only display a small sub-set of the colours capable of being captured, the sRGB standard was developed to squeeze the full range down to a range that could be viewed consistently on most monitors.
Given these limitations, I see no reason not to enable colour management within Firefox as you aren't losing anything if the colour space of the image being viewed is set to sRGB anyway. As Clive says, this is the standard for web images and the space that should be used if preparing images for web use.
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