> you cannot set a locally-hosted newtab page any longer

You can still set it with firefox.cfg, it's not not documented anywhere, and will break sometime in the future

  var {classes:Cc,interfaces:Ci,utils:Cu} = Components;
  var customNewTabURL = "file:///C:/File/Path/Here.html";
  Cu.import('resource:///modules/AboutNewTab.jsm');
  AboutNewTab.newTabURL = customNewTabURL;