This site will take forever to load or simply load only partially:
http://www.projo.com (The Providence Journal)
I used to have the same problem with CNN.com. It seems to me as if FireFox Portable has trouble loading content from different servers on one page, such as advertisements.
For example, if you go to projo.com you see the ads at the top of the page or on the left will load, but in the status bar at the bottom left of PortableFireFox, you will see either of two addresses Firefox is taking time to negotiate with: "spe.atdmt.com" or "ads.belointeractive.com".
The page will either stall there (with only the two ads displayed) or take 5 minutes to load completely. Like I said, I used to notice the same problem at CNN, but they have since made some adjustments to their page that don't cause the error any longer.