If you are monitoring your Internet connection activity, you may notice that Thunderbird sometimes automatically connects to the Internet. These connections are for routine maintenance, update and security purposes, which are described on this page.
Thunderbird makes a "loopback" connection to IP address 127.0.0.1 on non-Unix machines. That IP address is the local address of the machine where Thunderbird is running. This connection is not a security problem and should not be disabled. See the Firefox bug 100154 for more information.
Thunderbird periodically checks to see if updates are available for itself or for installed add-ons (extensions, themes, etc). To disable these checks:
By default, Thunderbird automatically loads a page with the latest Thunderbird information when the application is started. This page is downloaded from the Internet. To prevent Thunderbird from loading this page:
Each time the Add-ons tab is opened, Thunderbird prefetches a list of add-ons to improve responsiveness of the Get Add-ons pane. This connection is only made when the Add-ons Manager is opened.
Mozilla maintains a list of extensions that are not approved for use with Thunderbird. Thunderbird downloads this list from the Internet. To disable this:
Installed and activated extensions may connect to the Internet. For example, a translation extension might connect to the Internet to look up words in a dictionary. See the Thunderbird add-ons - frequently asked questions for information about disabling add-ons.