i know firefox uses a colorspace of its own, and if the site requires another one, it adapts. each site processes the images differently.
on one site i can't mention everything is darker for some reason.
on another site i can't mention - everything matches what it should look like printed out (We should do that here as well, it should lessen returns).
comparing my stuff from the clones of this site, they all seem to match. this site tends to reconfigure the color a little bit, usually adding a bit more contrast to it to make it pop more.
i always save in sRGB, it makes my life easier.
---Mike Savad
MikeSavad.com