Do you have colour management enabled in your browser.

[WillyPete]

The difference is stark!
Particularly with multi monitors, you can see the difference a monitor adds to what people see on the web with your images.

A blog about how to set it on Firefox 3.0 + here:
Color management support added to Firefox 3 - Mozilla Links

Color management is turned off by default to prevent subtle color variations affecting the overall look of web sites. To enable it you must set gfx.color_management.enabled to true (via about:config) and restart Firefox.

to set it to true, just double click the entry.
Just serch for "gfx" in about:config

warning: depending on your monitor's gamut, you might not be able to see TF's icons in the text boxes. As they said, subtle colour variations may be affected by this change.