Lightroom uses, and saves, color profile information to your JPEGs. But when you load those in Flickr, in Firefox, you lose the color profile and Firefox uses sRGB. So things look “off”.
What to do? Upgrade to Firefox 3(beta) and then go to about:config, and change:
gfx.color_management.enabled to true
(you can just double click on the config line)
Restart firefox and Viola! you now have color-matched graphics! …of course, you could also just buy a Mac.
Update: Apple also just released Safari 3.1, which also honors embedded color profiles (3.0.4 did, as well, but had a long standing ‘crashes on startup’ problem on all of my computers, so I can’t prove it.)