I'm making some PNGs for a web design and there's some strange color correction going on somewhere that's really getting on my nerves. Photoshop, Preview, and Safari are all displaying the graphics correctly, but in OmniWeb and Mozilla they are turning out noticeably brighter. I have color management turned off in Photoshop, so I assume my problem is that OmniWeb and Mozilla are doing some sort of fancy color correction.
Here is a side-by-side screenshot:

The colors at the top are correct: the three shades of gray should be #f0f0f0, #b0b0b0, #909090. The bottom two browsers display these colors as #f3f3f3, #bcbcbc, #a0a0a0. (Actually, I just turned on ColorSync in OmniWeb and it gets it right now; but instead, OW now fails most of the
browser gamma consistency tests.)
What I want is to make a graphic that will look the same in Photoshop, Preview, Safari, OmniWeb, Mozilla, and any other program, but this whole color management thing is confusing the heck out of me. Any suggestions?