I have this also in FF 52.9. Though I consider it common and expectable that software providers whitelist their own domains for phoning home, or even domains of associates/partners. I hadn't been even able to switch off auto-updating in uMatrix in its preferences, which I found very odd considering the nature of the extension. My solution: I block all firefox related domains not only by host file, but additionaly I block Mozialla IP ranges in iptables, plus I set any hyperlink from about:config to http://0.0.0.0[https://deref-gmx.net/mail/client/5WL2nsNuYlE/dereferrer/?redirectUrl=http%3A%2F%2F0.0.0.0]. And it is also educating to see there the amount (and context) of configured hosts, the browser connects by default. Extension updating / downloading I do manually only.
-- http://gnuzilla.gnu.org
