The ones that appear washed out in Firefox on my system show this when I look at them with EXIFtool:
Originally Posted by scott kirkpatrick
ProfileCopyright: Copyright (c) 2005-2007 Phase One A/S. All rights reserved.
ProfileClass: Input Device Profile
ProfileDateTime: 2007:08:10 18:51:51
The ones that render properly in Firefox on my system show this when I look at them with EXIFtool:
ProfileDescription: sRGB IEC61966-2.1
ProfileCopyright: Copyright (c) 1998 Hewlett-Packard Company
ProfileClass: Display Device Profile
ProfileDateTime: 1998:02:09 06:49:00
Both render properly in Safari (on OS X or iOS devices, likely Windows too but I don't have a Windows system to check against) because Safari is color-managed and honors the profiles. The same is true of Chrome, I believe, although I never run Chrome. Firefox does not color-manage and honor profiles by default.
You should always output image files you intend to post to the web with sRGB color calibration profile, not Adobe RGB (1998) or any other.