"Firefox Is Already Running but Is Not Responding"? Unlock Your Profile

Q: It happens every time I close Firefox.

Firefox isn't shutting down fully. Update it (Help → About Firefox) and check that no extension or antivirus is keeping it alive in the background.

Published ✓ Verified ⏱️ 2 min read July 30, 2025 · by Ryan Bennett

Firefox refuses to open and shows “Firefox is already running, but is not responding. The old Firefox process must be closed to open a new window.”

Why: Firefox needs an unlocked profile to start. If it didn’t shut down cleanly last time, a hidden firefox.exe process is still running and holding the profile’s lock file. Clear that and Firefox opens normally.

Fix 1: End the stuck Firefox process

Press Ctrl + Shift + Esc to open Task Manager.
On the Processes tab, find Firefox (press F to jump to it). Right-click it and choose End task — this closes every Firefox window at once.
Start Firefox normally.

If that doesn’t help, restart your computer. A reboot clears any stuck process and is the fastest fix if you can’t find one in Task Manager.

Fix 2: Delete the profile lock file

If the error persists after closing all processes, the lock file was left behind:

If you can open Firefox at all, go to menu (≡) → Help → More troubleshooting information, find Profile Folder, and click Open Folder. (If Firefox won’t open, browse to %APPDATA%\Mozilla\Firefox\Profiles\ and open your profile folder.)
Fully close Firefox, then delete parent.lock (on Windows; it may show as .parentlock).
Start Firefox.

While you’re there, right-click the profile folder, choose Properties, and make sure Read-only is unticked — a read-only profile can also trigger this error.

Fix 3: Create a new profile

If the lock won’t clear, the profile may be damaged:

Fully close Firefox, press Windows + R, type firefox.exe -P, and press Enter to open the Profile Manager.
Click Create Profile…, finish the wizard, and start Firefox with it.
Copy your bookmarks and passwords across from the old profile folder once Firefox is working again.

FAQ

It happens every time I close Firefox. Firefox isn’t shutting down fully. Update it (Help → About Firefox) and check that no extension or antivirus is keeping it alive in the background.

I don’t see Firefox in Task Manager but still get the error. Restart the PC (Fix 1), then delete the lock file (Fix 2) — the lock can outlive a process that already exited abnormally.

Sources: Mozilla Support — “Firefox is already running but is not responding” error - How to fix