I haven't heard that particular complaint before. Are you by chance thinking of under saturated colors? Some people on Flickr have been uploading jpgs with in the AdobeRGB color profile. When this profile is viewed in most web browsers the colors are displayed as though they were sRGB. Firefox 3 now has the ability to be set to display pictures with the AdobeRGB color profile as true AdobeRGB colors.
To set Firefox to do this, open a new tab, type in "about:config", find the entry titled "gfx.color_management.enabled", double-click (so it becomes enabled), and restart Firefox. Now any images on flickr or any other site with the AdobeRGB profile with be shown correctly.