Firefox 21 was just released and introduced a new feature which submits data to Mozilla by default. Here is to disable it.
How to Turn Off The New Health Report Feature in Firefox on Windows
Open Firefox and click on the “Firefox” menu, from here expand the Options menu item and select Options again.
You will then need to switch over to the Advanced section.
Here you will need to switch over to the Data Choices tab and then uncheck the option to “Enable Firefox Health Report” as seen in the screenshot below.