A Firefox add-on called ShowIP
has been sending the URLs of pages the user has visited to a marketing company without displaying a warning even if the user was surfing in "Private Browsing" mode. More on the story here: Firefox add-on exposes visited URLs
For users who want to see the IP of the site they're visiting, I suggest installing this add-on instead which achieves the same goal and more: Flagfox
It adds the country flag of the page you're on to the end of the Firefox Location Bar. Hovering the mouse over that reveals the IP address and country location. Similarly, right clicking the flag allows the user to check WHOIS, WOT and various other useful functions.