Can help you with that one...
Safari supports multiple colour spaces - it does this by accessing the ColorSync libraries as part of OS X.
So... Safari can correctly display both sRGB and AdobeRGB colourspaces.
If you were shooting in RAW (and most likely as AdobeRGB or other variant), and then opened the image in photoshop, edited, then saved (either via save as or save for web) the colourspace has been preserved as say AdobeRGB.
Firefox on the other hand will display images as sRGB - you'll notice this as a huge desaturation of your carefully processed images.
To avert this, before "Saving As" or "Save for Web" select
Image menu -> Mode -> Convert to Profile
The dialog box will says something like:
Source Space: Adobe RGB (1998)
Destination Space: Working RGB sRGB IEC61966-2.1
Engine: Adobe (ACE)
Intent: Perceptual
(selected) Use Black Point Compensation
Now save your JPEG out again, and test it in Safari and Firefox. Both should look as close as they're going to get
