Firefox 'already running' after crash – fix profile lock

When this error shows up

You just rebooted your PC after a blue screen, power outage, or forced shutdown. You click Firefox and instead of your tabs loading, you get this: “Firefox is already running but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system.”

You check Task Manager (Ctrl+Shift+Esc) – no Firefox processes listed. You try again. Same message. It's not a ghost process. It's a leftover lock file.

Why it happens

Firefox uses a file named parent.lock inside your profile folder to prevent two instances from writing to the same data at the same time. Normally, Firefox deletes this file when you close it. But if your system crashes hard – like a kernel panic on Windows (BSOD), a hard reset, or a power loss – Firefox doesn't get a chance to clean up. That lock file stays behind. The next time Firefox starts, it sees that file and thinks another copy is already running. So it refuses to open.

It's not profile corruption in the scary sense – your bookmarks, passwords, and history are still intact. It's just a single leftover file causing the block.

The fix: delete the lock file

Here's the exact sequence. I've done this on hundreds of machines. It works every time unless your profile is actually corrupted (which is rare after a simple crash).

1. Open your Firefox profile folder.
   • Windows 10/11: Press Win+R, type %APPDATA%\Mozilla\Firefox\Profiles\, press Enter. You'll see one or more folders ending in .default-release or just .default. Pick the one that looks like a random string (e.g., abcd1234.default).
   • macOS: In Finder, press Cmd+Shift+G, type ~/Library/Application Support/Firefox/Profiles/, press Go. Pick your profile folder.
   • Linux (Ubuntu/Fedora/etc.): Open a terminal, type cd ~/.mozilla/firefox/, then ls. You'll see folders like xyz.default. Navigate into that one.

   After this step, you should be inside a folder with files like places.sqlite, logins.json, and – the troublemaker – parent.lock.

2. Delete only the parent.lock file.
   • Look for a file named parent.lock (no icon, typically 0 bytes in size).
   • Right-click it and choose Delete. Windows might ask for admin permission – click Yes.
   • If you don't see it, make sure hidden files are visible:
     • Windows: In File Explorer, click View → Show → Hidden items.
     • macOS: Press Cmd+Shift+. in the Finder window.
     • Linux: In your file manager, press Ctrl+H.

   After deleting, you should see the file is gone. Don't touch anything else – especially not places.sqlite (that's your browsing history) or logins.json (that's stored passwords).

3. Start Firefox normally.
   • Double-click the Firefox icon on your desktop or taskbar.
   • It should open straight away. If you had tabs open before the crash, a prompt will ask if you want to restore them – click “Restore Previous Session”.

   After starting, Firefox will create a fresh parent.lock file automatically. That's fine – it's supposed to be there while Firefox is running. The problem was the leftover one.

What to do if it still shows the error

Sometimes the lock file is stubborn, especially on Windows if Firefox crashed with a process that's actually still hanging in the background.

• Kill Firefox processes in Task Manager. Press Ctrl+Shift+Esc, look under Processes for “Firefox” (not just “Mozilla Firefox”). Right-click and choose “End task”. Try step 2 again.
• Reboot again. A full restart clears any orphaned system handles. Then repeat the steps from the top.
• Check for a second profile folder. If you have more than one profile folder in the Profiles directory, you might have opened the wrong one. Firefox usually uses the one with “.default” in the name. Try the other one.
• If nothing works, rename your profile folder as a last resort. Go up one level to the Profiles folder, rename the whole profile folder to something like old-profile, then start Firefox. It will create a fresh profile. Your old data is still in that renamed folder – you can later move bookmarks and passwords out of it using the Firefox Library.

That's it. This fix takes 30 seconds once you know where to look. I've never seen a simple crash corrupt the actual Firefox profile – it's almost always just that one lock file.