It could be checking for things like updates. Mozilla uses (or has) Edgecast, Akamai and also Amazon cloudfront (as needed) for cloud service. Mozilla uses a [https://developer.mozilla.org/en-US/docs/Glossary/CDN CDN] or Content Delivery Network as they no longer use [http://www-archive.mozilla.org/mirrors.html mirrors] as they did before. https://en.wikipedia.org/wiki/Akamai_Technologies Thunderbird is however not a Mozilla product anymore though Mozilla does help host downloads for Thunderbird and provides a place on this SUMO site for community support.