The problem here seems to be that the following [JavaScript error, which can be seen in the logs](https://treeherder.mozilla.org/logviewer?job_id=472814717&repo=autoland&lineNumber=171176, causes breakage in the startup phase causing Firefox not correctly to initialize. ``` JavaScript error: chrome://browser/content/sidebar/browser-sidebar.js, line 1532: TypeError: can't access property "getAttribute", arrowScrollbox is null ``` Because the above error isn't caught all the remaining startup code isn't run, and as result no `browser-idle-startup-tasks-finished` notification is sent out, which is required to initialize Marionette.
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The problem here seems to be that the following [JavaScript error, which can be seen in the logs](https://treeherder.mozilla.org/logviewer?job_id=472814717&repo=autoland&lineNumber=171176), causes breakage in the startup phase causing Firefox not correctly to initialize. ``` JavaScript error: chrome://browser/content/sidebar/browser-sidebar.js, line 1532: TypeError: can't access property "getAttribute", arrowScrollbox is null ``` Because the above error isn't caught all the remaining startup code isn't run, and as result no `browser-idle-startup-tasks-finished` notification is sent out, which is required to initialize Marionette.