I think the answer to your problem is in the link I gave you in my last reply, if you read it carefully. Here's what I think is going on.
The example image on your computer is in sRGB, and it is TAGGED. Firefox 3.5.2, and Safari, are color managed browsers that recognize the tag--and just like Photoshop, they display it correctly on any monitor.
That same example image in sRGB was stripped of its tag when it was uploaded to the web, so it is now UNTAGGED. The Windows operating system assumes untagged images are in sRGB space, so they display correctly in a color managed browser. But the Mac operating system assumes untagged images are in your monitor's color space, and assigns your monitor's profile--in the case of your calibrated wide-gamut monitor, close to Adobe RGB 1998. The browser displays them as if you had (incorrectly) assigned an aRGB tag.
You can test this by opening your example image in PS, then saving a JPEG version in which you uncheck the box beside "ICC profile: sRGB...", so that the tag is stripped. Now drag that untagged image onto your Firefox or Safari icon; it will probably look the same as the version on your web site: oversaturated.
If this is indeed the case, there seems currently to be no solution. Images that are in sRGB but are untagged will display incorrectly on wide gamut monitors under the Mac operating system. You can make sure that images on your own web site look OK by always keeping them tagged. But you can't control what others do, and most web images are untagged, so they won't look right in your browser on your monitor.