This can be caused by a problem with the color profile for your display monitor or color profiles embedded in images.
You can disable color management to test that.
You can set the pref gfx.color_management.mode to 0 on the about:config page to disable Color Management.
You need to close and restart Firefox to make the change effective.
See http://kb.mozillazine.org/gfx.color_management.mode

See https://developer.mozilla.org/En/ICC_color_correction_in_Firefox

This solution solves the problem. But why isn't color management set to 0 by default??? I bet the majority of users never even find this function.

I had the same problem. Photos on facebook (and on some other sites) were significantly darker only when viewed with Firefox (both on 4.0 and 3.6.16) but not with Internet Explorer. Dark faces on photos viewed with Firefox were often unrecognisable, unless you saved the photo and then opened it with some other photo viewer.

Obviously the problem had nothing to do with the display monitor if other browsers or photo editors can show the same picture on the same monitor normally. (I'm using a normal separated LCD monitor and Windows XP.)

However - on some laptops there seemed to be no problem - as even Firefox (both 4.0 and 3.6.16) showed pictures normally.

It seems quite probable that approximately one half of Firefox users has this problem if they don't change the "pref gfx.color_management.mode to 0".

I think the next update should return this value to 0 by default.

The real solution is to make the color management on/off option visible to the normal end user, and to educate users about color management. If color management is turned ON (and working, which I don't think it is in FF4), then colors should come out like they are meant to unless an incorrect color profile has been installed on the system.

BTW, color management on/off needs to affect images immediately (at least when reloaded) to allow people to understand what's going on. Not needing a restart of Firefox like today.