Adobe Lightroom and color spaces
Here is an interesting article from CNET on color spaces, I have to admit I did not fully understand the differences between all of the choices within Lightroom. I have been using Lightroom for several months now and really love this application, it is not cheap but then you do get what you pay for.
That was my problem. I was exporting to JPEGs with an Adobe RGB color space. Lightroom (like Photoshop and other higher-end imaging programs) understand how to properly display images using that color space on an sRGB monitor. However, neither Firefox nor IrfanView do.
Moral of the story? Although Apple's Safari and the Firefox 3 beta can display non-sRGB color spaces correctly, Microsoft's Internet Explorer and Firefox 2.x don't. Nor do most basic file viewers. Therefore, for most purposes--and certainly for Web display--it's best to use JPEGs with an sRGB color space or the results probably won't be what you expect.
On an additional Lightroom note, with version 1.3 and the new SDK, and thanks to Jeffrey Friedl you can now export directly to SmugMug. Jeffrey is still working through some bugs, but recent versions are starting to work pretty well. Thanks Jeffrey!