Photoshop :: CS 5.1 - Save For Web Preview Isn't Accurate With Browser

Using PS CS5.1, converting a large space image to sRGB for web use does not give accurate render in actual browser. This is a common problem, but usually there are a couple of simple user errors that lead to this, and it usually means a less saturated image. This, however, seems unlikely to be user error, as I've tested it with a freshly created, sRGB image, with simple gradient, outputting it with embedded profile (non-embedded gives identical results in browser), and it becomes overly saturated.
 
The interesting, and probably all telling thing, is that viewing the optimized web version with Monitor Color selected in the preview gives what my browser actually shows me. Likewise, soft proofing using my Monitor RGB gives the same image as my browser.
 
I'm on WinXP x64. Browser is Firefox, and I've tried all three color management settings with no change, and also same results with Chrome browser.
 
This grad is in the ProPhoto space, converted during save for web. Color is a good match, with a slight shift, not unexpectedly. Note, preview is with Internet Standard RGB.

Same grad, this time with the preview set to Monitor Color. There is quite a  saturation increase. This is what my browser actually shows me. I do not have desaturate colors selected in my color settings.