Investigate update on Windows via BITS
It seems there is coming a new update method for Windows users with Firefox 68 ESR which is called BITS (Background Intelligent Transfer Service), which is a Windows component. The marketing promise is that "This change will allow Firefox to continue downloading an update after Firefox has been closed."  which seems to be dangerous in the Tor Browser context.
There is a pref we can flip, though to use the older internal updater . However, we should make sure the potential proxy bypass I am seeing here is actually mitigated by that.
 https://www.ghacks.net/2019/06/24/firefox-will-use-bits-on-windows-for-updates-going-forward/  https://groups.google.com/forum/#!topic/mozilla.dev.platform/PCzoYCfi_fk  https://bugzilla.mozilla.org/show_bug.cgi?id=1553977