I can reproduce the error from comment 2 in Firefox 69 - 74. The error points to https://searchfox.org/mozilla-central/rev/c52d5f8025b5c9b2b4487159419ac9012762c40c/toolkit/actors/BrowserElementParent.jsm#81 In Firefox 68, the test case from the initial report does not trigger any errors. The new error is probably due to the change in bug 1533949 . If I add `event.stopPropagation()` at https://searchfox.org/mozilla-central/rev/c52d5f8025b5c9b2b4487159419ac9012762c40c/toolkit/components/extensions/ext-browser-content.js#161 , then the error message isn't printed any more. There is also another error message when the popup is closed (or when the mouse pointer hovers above the extension button and then away): > [Exception... "Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsIControllers.removeController]" nsresult: "0x80004005 (NS_ERROR_FAILURE)" location: "JS frame :: chrome://global/content/elements/browser-custom-element.js :: destroy :: line 1374" data: no] There doesn't seem to be any bad side effect besides these errors.
Bug 1394019 Comment 3 Edit History
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I can reproduce the error from comment 2 in Firefox 69 - 74. The error points to https://searchfox.org/mozilla-central/rev/c52d5f8025b5c9b2b4487159419ac9012762c40c/toolkit/actors/BrowserElementParent.jsm#81 In Firefox 68, the test case from the initial report does not trigger any errors. The new error is probably due to the change in bug 1533949 . If I add `event.stopPropagation()` at https://searchfox.org/mozilla-central/rev/c52d5f8025b5c9b2b4487159419ac9012762c40c/toolkit/components/extensions/ext-browser-content.js#161 , then the error message isn't printed any more. There is also another error message when the popup is closed (or when the mouse pointer hovers above the extension button and then away): > [Exception... "Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsIControllers.removeController]" nsresult: "0x80004005 (NS_ERROR_FAILURE)" location: "JS frame :: chrome://global/content/elements/browser-custom-element.js :: destroy :: line 1374" data: no] There doesn't seem to be any bad side effects besides these errors.