Disable Firefox Hotfix updates
I'm not sure exactly when this was added, but we should fix it soon, so I'm tagging it as a regression.
The pref extensions.hotfix.id specifies the ID of a magic hotfix addon for emergency Firefox releases that can be fixed with simple pref or addon-based XPCOM changes. That is a nice property, except that the addon may make assumptions about Firefox 38-ESR and its prefs that don't apply to TBB. It's probably not safe for us to have Mozilla's hotfixes go to our users for this reason.
Simply emptying the pref should disable the addon update pings.