There again you come with this "free software can't give you privacy" bs...
yeah you did a tcpdump and found some background chatting, which is not nice,
Do the following:
1. download the source code of firefox
2. do a sed replace for every unwanted URL firefox is communicating with and
substitute it with "localhost"
3. compile firefox
=> you solved the problem for yourself
4. release your changes to the public
=> you solved the problem for everybody else
Free Software DOES give you control, and control enables you to get privacy.
Actually you proved it yourself but didn't realize it.