Firefox

3rd party cookies
Cookies are pieces of information that a website can store on your computer. There are two types of cookies: Go to Preferences, and then, in the Privacy section, set Accept third-party-cookies to Never.
 * First party cookies: these are the cookies from the site you are visiting. They are used for example for the log-in functionality of most websites.
 * Third party cookies: they are set by other website. For example, any time you visit a website with a like button from Facebook, Facebook will set a cookie on your computer. This allows them to follow them on most of the website you are going to. This is used by advertiser to know the websites you go to and show you the same advertising on all websites. In practice, very few websites use this for functionalities. It is quite safe to disable them.



WebRTC
WebRTC is a protocol that allows peer to peer communication between browsers. This is for example for audio/video chat. To allow faster connections to computer within the same local network, this protocol allows the browser to share all your local IP addresses.

This has two major problems: To stop the leak, go to  and change this setting
 * If a website contain malware, knowing your addresses is helping it to infect your network.
 * This can be used to fingerprint your device and track you around the web.