bring the patch closer to the then-upstream comm-central. might have the
negative side-effect of being detectable by websites according to:
https://developer.mozilla.org/en-US/docs/Mozilla/Chrome_Registration
this hopefully makes the monkey patch update persistent.
Note: instead of using FileUtils, this would be another method:
const {Services} = Cu.import('resource://gre/modules/Services.jsm');
let manifest = Services.dirsvc.get('GreD', Ci.nsIFile);
manifest.append('legacy.manifest');