Found while investigating Bug 1392531. STRs: (note, if you are on latest central, you need the patches from Bug 1392531 to be applied first, but I checked and the issue also occurs *before* the regression that the patches are fixing) - use a clean profile - install ReactDevTools from https://addons.mozilla.org/en-US/firefox/addon/react-devtools/ - open new tab (on any page, doesn't matter) - open devtools - close devtools - open the browser toolbox - go to the debugger ER: Debugger UI should be displayed AR: Debugger UI is empty Errors logged: Full message: TypeError: can't access dead object Full stack: createNonSourceMappedActor@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/server/actors/utils/TabSources.js:299:1 createSourceActors/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/server/actors/utils/TabSources.js:383:19 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 Promise.prototype.then@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:455:5 createSourceActors@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/server/actors/utils/TabSources.js:382:12 _discoverSources/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/server/actors/script.js:1338:14 _discoverSources@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/server/actors/script.js:1337:16 onSources@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/server/actors/script.js:1343:12 onPacket@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/server/main.js:1797:15 _onJSONObjectReady/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/transport/transport.js:483:11 exports.makeInfallible/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/ThreadSafeDevToolsUtils.js:109:14 exports.makeInfallible/<@resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/shared/ThreadSafeDevToolsUtils.js:109:14

I missed a detail in my STRs, when opening DevTools, you should switch to the debugger first. STRs are: - use a clean profile - install ReactDevTools - open new tab - open devtools - switch to Debugger <---- New step! - close devtools - open the browser toolbox - go to the debugger Some additional information: - disabling the react devtools extension makes the issue disappear - I don't know if the issue is specific to React devtools' extension. It can probably be triggered with other extensions too.

Moving to the backlog as we've clearly not been able to get to this quickly, so this isn't a P2.