Here's the stack: Grid_resizeGrid@chrome://browser/content/newtab/newTab.js:847:9 Grid_handleEvent@chrome://browser/content/newtab/newTab.js:699:9 EventListener.handleEvent*Grid_init/<@chrome://browser/content/newtab/newTab.js:671:7 executeCallbacks@resource://gre/modules/NewTabUtils.jsm:896:13 Links_populateCache/<@resource://gre/modules/NewTabUtils.jsm:908:11 _populateProviderCache/cache.populatePromise</</<@resource://gre/modules/NewTabUtils.jsm:1085:11 DirectoryLinksProvider_getLinks/<@resource:///modules/DirectoryLinksProvider.jsm:655:7 process@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:922:23 walkerLoop@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:806:7 Promise*scheduleWalkerLoop@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:739:11 schedulePromise@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:770:7 completePromise@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:707:7 onSuccess@resource://gre/modules/osfile/osfile_native.jsm:63:7

This doesn't affect the Firefox UI since it's in the content area.