Try to rename/remove xulstore.json and when present delete localstore.rdf in the Firefox profile folder with Firefox closed.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Hi cor-el,

Thank you for explaining how to stop it happening, I'll try that when I'm back on that computer.

However I'm actually more interested in why it started happening. What could cause this? An add-on? A webpage? A new build of Firefox? Something malicious?

Thanks :)

Some windows are persistent across a session and are opened automatically on a next start. This might be happening in your case and xulstore.json stored all window information, so renaming or removing this file might help to correct this.

Hi cor-el, Thanks for explaining. So if I delete that file what else might change? Will I lose all my settings? My tabs? Thanks.

The xulstore.json file only stores information about open windows (dimensions, position, column order) and some other data like the collapse/expanded state of bookmark folders.

I checked xulstore.json and I might have been wrong and that your issue is not with xulstore.json, but is a problem with sessionstore.jsonlz4 I see these key values in the sessionstore file:

"browserConsole":false,"browserToolbox":false,"scratchpads":[]

When I close Firefox with a console open then these items get a true value and the console is reopened on the next start. So if the Browser Console opens automatically then this means that in your case the open state true is stored.

Is the sessionstore.jsonlz4 updated when you close Firefox if you check its time stamp?

If this file is corrupted then you will have to rename this file and start with a fresh file. The sessionstore.jsonlz4 file, that is only present when Firefox is closed, stores session data like your pinned tabs and open and closed tabs and windows, so you have to make a note or bookmark your tabs to be able to restore them .

See also this tool that you can use to inspect a compressed sessionstore file.

• https://www.jeffersonscher.com/res/scrounger.html

Hi @cor-el, Thanks for the explanation and extra suggestions. Deleting xulstore.json has no effect. Unfortunately, neither did deleting sessionstore.jsonlz4 - and I did also rename my sessionstore-backups folder to ensure it would not recover the session from their either. I tried editing sessioncheckpoints.json for the setting about restoring windows and also checked all my preferences. Nothing prevented the browser console from opening. What else could be causing this? Cheers :)

Hi everyone, I still have this problem, if anyone can help I would appreciate it! Thanks :)

Are there usual messages in the Browser Console?

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Hi @cor-el,

Well, yes I suppose it's full of errors! These are the messages when I first opened Firefox just now.

There are a few that say this: Unchecked lastError value: Error: First-Party Isolation is enabled, but the required 'firstPartyDomain' attribute was not set. heuristicblocking.js:154

Note that I have privacy.firstparty.isolate and privacy.firstparty.isolate.restrict_opener_access set to true, and privacy.firstparty.isolate.block_post_message is set to default (I think this is a new attribute, it wasn't there a few versions ago when I set firstpartyisolate). I have these options set on my work computer as well but the browser console doesn't appear on that.

There are also several messages that say this: Exception ​ columnNumber: 0 ​ data: null ​ filename: "resource://devtools/server/actors/network-monitor/network-response-listener.js" ​ lineNumber: 335 ​ location: XPCWrappedNative_NoHelper { QueryInterface: QueryInterface(), filename: Getter, name: Getter, … } ​ message: "Component returned failure code: 0x80040111 (NS_ERROR_NOT_AVAILABLE) [nsICacheInfoChannel.isRacing]" ​ name: "NS_ERROR_NOT_AVAILABLE" ​ result: 2147746065 ​ stack: "NetworkResponseListener.prototype._getSecurityInfo<@resource://devtools/server/actors/network-monitor/network-response-listener.js:335:26\nexports.makeInfallible/<@resource://devtools/shared/ThreadSafeDevToolsUtils.js:111:22\nonStartRequest@resource://devtools/server/actors/network-monitor/network-response-listener.js:224:10\n" ​ <prototype>: ExceptionPrototype { toString: toString(), name: Getter, message: Getter, … } ThreadSafeDevToolsUtils.js:90:13

I don't have any idea what that's referring to.

Also one count of this: [Exception... "Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsIDOMWindowUtils.removeSheet]" nsresult: "0x80004005 (NS_ERROR_FAILURE)" location: "JS frame :: resource://gre/modules/ExtensionCommon.jsm :: runSafeSyncWithoutClone :: line 75" data: no] ExtensionCommon.jsm:75:12

And one count of this: uncaught exception: Object

And this: TypeError: can't access dead object

I'll try the other stuff in a minute. Cheers :)

The browser console didn't appear in safe mode. Theme was already set to default.

The messages in the browser console seem to be different each time I restart Firefox. They generally seem to be about the different anti-tracking methods (supercookie.js, dnt.js, socialwidgets.js, etc).