There is a problem with Firefox displaying a grayscale image with the sRGB chunk, when color management is enabled. See bug #403423 at bugzilla.mozilla.org. The images below are, from left to right,

• The original image from pmt.sf.net/gamma_test, with only the sRGB chunk
• The same, with a gAMA chunk added
• The same with only the gAMA chunk
• The original image, but as a color_type 2 PNG.

sRGB alone

sRGB and gAMA

gAMA alone

sRGB alone ct2

The trouble seems to be the combination of grayscale and sRGB.