From MozillaZine Knowledge Base
The following is a list of all connections that Firefox may establish at application startup and how to stop them.
- A loopback connection (to IP address 127.0.0.1) on non-Unix machines [1]. The browser is communicating normally with itself, and it is not recommended this be blocked.
- Loading Live Bookmarks. Deleting all Live Bookmarks will stop these connections.
- Checking for application updates. "Tools -> Options -> Advanced -> Update" provides options to disable update checks.
- Loading home page. Setting the homepage to a blank page or a local page will stop this connection.
- Updating phishing sites database. Setting browser.safebrowsing.enabled in about:config to false will disable this feature.
- Prefetching links if the page it's on defines any. See the Prefetch FAQ and network.prefetch-next for further information. To disable pings in prefetched links, see browser.send_pings.
- Updating the extension blacklist. Setting extensions.blocklist.enabled to "false" in about:config will disable this feature.
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