While not fatal, once of the recent Style Inspector landings from over the weekend caused these errors to appear in the logs. https://tbpl.mozilla.org/php/getParsedLog.php?id=26702280&tree=Fx-Team TypeError: inspector.once(...) is undefined: simpleInherit@chrome://mochitests/content/browser/browser/devtools/styleinspector/test/browser_ruleview_inherit.js:27 TypeError: inspector.once(...) is undefined: emptyInherit@chrome://mochitests/content/browser/browser/devtools/styleinspector/test/browser_ruleview_inherit.js:60 TypeError: inspector.once(...) is undefined: elementStyleInherit@chrome://mochitests/content/browser/browser/devtools/styleinspector/test/browser_ruleview_inherit.js:79 TypeError: inspector.once(...) is undefined: SI_inspectNode@chrome://mochitests/content/browser/browser/devtools/styleinspector/test/browser_styleinspector_bug_672744_search_filter.js:36 11:20:32 INFO - TEST-INFO | chrome://mochitests/content/browser/browser/devtools/styleinspector/test/browser_ruleview_inherit.js | Console message: [JavaScript Warning: "XUL box for hbox element contained an inline span child, forcing all its children to be wrapped in a block." {file: "resource://gre/modules/commonjs/toolkit/loader.js -> resource:///modules/devtools/inspector/highlighter.js" line: 470}] 11:20:32 INFO - WARNING: Subdocument container has no frame: file ../../../layout/base/nsDocumentViewer.cpp, line 2380 11:20:33 INFO - ++DOMWINDOW == 494 (0x16c778e78) [serial = 13238] [outer = 0x16c7d4cc8] 11:20:33 INFO - ++DOMWINDOW == 495 (0x16c7563d8) [serial = 13239] [outer = 0x16c7c8098] 11:20:33 INFO - ++DOMWINDOW == 496 (0x16c72b628) [serial = 13240] [outer = 0x16c7b7ce8] 11:20:33 INFO - ++DOMWINDOW == 497 (0x16c70f6e8) [serial = 13241] [outer = 0x16c7aeec8] 11:20:33 INFO - TypeError: inspector.once(...) is undefined: simpleInherit@chrome://mochitests/content/browser/browser/devtools/styleinspector/test/browser_ruleview_inherit.js:27 11:20:33 INFO - @chrome://mochitests/content/browser/browser/devtools/styleinspector/test/head.js:57 11:20:33 INFO - @resource://gre/modules/commonjs/toolkit/loader.js -> resource:///modules/devtools/shared/event-emitter.js:63 11:20:33 INFO - EventEmitter_emit@resource://gre/modules/commonjs/toolkit/loader.js -> resource:///modules/devtools/shared/event-emitter.js:107 11:20:33 INFO - ToolSidebar_addTab/onIFrameLoaded<@resource://gre/modules/commonjs/toolkit/loader.js -> resource:///modules/devtools/framework/sidebar.js:80 11:20:33 INFO - TEST-INFO | chrome://mochitests/content/browser/browser/devtools/styleinspector/test/browser_ruleview_inherit.js | Console message: [JavaScript Error: "TypeError: inspector.once(...) is undefined: simpleInherit@chrome://mochitests/content/browser/browser/devtools/styleinspector/test/browser_ruleview_inherit.js:27 11:20:33 INFO - @chrome://mochitests/content/browser/browser/devtools/styleinspector/test/head.js:57 11:20:33 INFO - @resource://gre/modules/commonjs/toolkit/loader.js -> resource:///modules/devtools/shared/event-emitter.js:63 11:20:33 INFO - EventEmitter_emit@resource://gre/modules/commonjs/toolkit/loader.js -> resource:///modules/devtools/shared/event-emitter.js:107 11:20:33 INFO - ToolSidebar_addTab/onIFrameLoaded<@resource://gre/modules/commonjs/toolkit/loader.js -> resource:///modules/devtools/framework/sidebar.js:80 11:20:33 INFO - " {file: "resource://gre/modules/commonjs/toolkit/loader.js -> resource:///modules/devtools/shared/event-emitter.js" line: 112}] 11:20:33 INFO - ++DOCSHELL 0x16c455ea0 == 97 [id = 5181] 11:20:33 INFO - ++DOMWINDOW == 498 (0x16c437878) [serial = 13242] [outer = 0x0] 11:20:33 INFO - WARNING: NS_ENSURE_TRUE(NS_SUCCEEDED(rv) && subjPrincipal) failed: file ../../../docshell/base/nsDocShell.cpp, line 8393 11:20:33 INFO - line: 11:20:33 INFO - console.error: 11:20:33 INFO - element changed 11:20:33 INFO - console.error: 11:20:33 INFO - element changed 11:20:33 INFO - line: 11:20:33 INFO - line: background-color: green; 11:20:33 INFO - line: color: purple; 11:20:33 INFO - line: 11:20:33 INFO - TEST-PASS | chrome://mochitests/content/browser/browser/devtools/styleinspector/test/browser_ruleview_inherit.js | Should have 2 rules. 11:20:33 INFO - TEST-PASS | chrome://mochitests/content/browser/browser/devtools/styleinspector/test/browser_ruleview_inherit.js | Element style attribute should not consider itself inherited. 11:20:33 INFO - TEST-PASS | chrome://mochitests/content/browser/browser/devtools/styleinspector/test/browser_ruleview_inherit.js | Inherited rule should be the one that includes inheritable properties. 11:20:33 INFO - TEST-PASS | chrome://mochitests/content/browser/browser/devtools/styleinspector/test/browser_ruleview_inherit.js | Rule should consider itself inherited. 11:20:33 INFO - TEST-PASS | chrome://mochitests/content/browser/browser/devtools/styleinspector/test/browser_ruleview_inherit.js | Should only display one inherited style 11:20:33 INFO - TEST-PASS | chrome://mochitests/content/browser/browser/devtools/styleinspector/test/browser_ruleview_inherit.js | color should have been inherited. 11:20:33 INFO - TypeError: inspector.once(...) is undefined: emptyInherit@chrome://mochitests/content/browser/browser/devtools/styleinspector/test/browser_ruleview_inherit.js:60 etc

I won't be around to shuffle this through (on vacation), but it's pretty straightforward. I was adding .then() clauses to the return value of once(). They weren't necessary.