Firefox 63 changed the locking of sqlite files to "exclusive mode" to prevent issues on network drives - the sqlite files are now locked by default.

Are you sure another application isn't trying to access this file, preventing Firefox from accessing it? We've had reports that "Cookie" is one application that can interfere.

I'm not 100% sure, but I can't think of any application accessing the file. The machine is pretty naked. We also uninstalled Firefox, deleted all data and reinstalled it again just to get the error again when opening Firefox for the first time. So, if "Cookie" is some kind of extension, it couldn't have been installed at that time.

Interestingly, the user homes are stored on a network drive mounted on login via SMB. But if this has something to do with the error?

We have several of those machines that are more or less equal. I'll try to have the user use another machine to check whether this is related to the machine, the user's profile or the combination on both. Until now only one user complained about this problem, so I guess that is is somehow machine related. But I'll test this.

The user tested his profile on another machine. Firefox worked without any error messages. The problem seems to be related to one particular machine. But how to diagnose it?

Was the whole profile copied to the other machine?

If so, let's try this; Delete the profile on the original system. Then Copy the working profile from the other back to the original. Does it work now?

FredMcD said

Was the whole profile copied to the other machine?

Not really. All users have a roaming profile, which resides on a NAS and is mounted on login. So the profile on the second machine is the exact same profile that is used on the machine producing the error.

I see.

Let’s do a full clean re-install;

Download Firefox For All languages And Systems {web link}

Save the file. Then Close Firefox.

Using your file browser, open the Programs Folder on your computer.

Windows: C:\Program Files C:\Program Files (x86)

Mac: Open the "Applications" folder. https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac

Linux: Check your user manual. If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it. See Install Firefox on Linux; https://support.mozilla.org/en-US/kb/install-firefox-linux

If you downloaded and installed the binary package from the Firefox download page, simply remove the folder Firefox in your home directory. http://www.mozilla.org/firefox#desktop ++++++++++++++++++++++++++++ Look for, and remove any Mozilla or Firefox program folders. Do not remove the Mozilla Thunderbird folder if there is one.

Do Not remove any profile folders.

After rebooting the computer, run a registry scanner if you have one. Then run the installer. +++++++++++++++++++++++++++ If there is a problem, start your Computer in safe mode and try again.

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode This shows how to Start all Computers in Safe Mode; Free Online Encyclopedia

Thanks for the detailed answer. However, we already tried to reinstall Firefox. One time with the user's profile, one time also without to have a fresh profile created on first start. Both times we got the error again.

Unfortunately, I screwed the original question's markup, so this list of things we already tried wasn't that easy to read. Sorry for that.

To me it looks like there is something on that particular computer that interferes with Firefox. But Firefox doesn't tell what it is.

Well, we wouldn't want this to be easy, right?

Have you tried to Start your Computer in safe mode with network support. Then start Firefox. Try Secure websites. Is the problem still there?

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode Starting Any Computer In Safe Mode; Free Online Encyclopedia

FredMcD said

Have you tried to Start your Computer in safe mode with network support. Then start Firefox.

We did, the same error.

FredMcD said

Try Secure websites. Is the problem still there?

Well, the error message is already displayed when Firefox starts (with a blank page). We don't have to open any website to get the error message.

Did you check the permissions for the profile folder(s)?

• https://appletoolbox.com/2016/07/fix-corrupt-user-accounts-macos/

• ~/Library/Application Support/Firefox/Profiles/<profile>/
• ~/Library/Caches/Firefox/Profiles/

In Mac OS X v10.7 and later, the ~/Library folder in the Home directory is a hidden folder.

• http://kb.mozillazine.org/Show_hidden_files_and_folders#Mac_OS_X