Actual URLs are treated very carefully. The only ways I know that they could be leaked: (1) there is limited exposure via the SafeBrowsing stuff, and (2) if you crash then URLs could be swept up in the crash report and visible to privileged people at Mozilla. Or if you manually submit a profile and don't anonymize it (there's a checkbox to submit URLs that is checked by default), but you have to go out of your way to do that. There very well could be other ways, but browsing history is treated as very sensitive data for both legal and moral reasons.

Personally, I would not trust Mozilla to be unable to answer the question "has this IP address ever used Firefox?", especially if under subpoena. The IP is temporarily in the server logs, it's used for coarse location information, etc. I doubt anyone is all that interested in asking that question, though.