Firefox Is Already Running but Is Not Responding, or A Copy of Firefox Is Already Open: How to Fix It

Updated: 8 October 2026 · Applies to: Firefox 157 on Linux (tested headless on Debian 13), snap Firefox on Ubuntu 24.04 and 26.04, Firefox on Windows and macOS (steps according to Mozilla)

Firefox shows "Firefox is already running, but is not responding" (on macOS: "A copy of Firefox is already open") when its profile is locked. Almost always another Firefox process is still running with that profile, often with no window visible. Close every Firefox process first; only if the message stays, delete the lock files in the profile folder. On Linux:

pgrep -l firefox      # is Firefox still running?
pkill firefox         # ask it to quit
pgrep -l firefox      # wait 10 seconds, then check again

What the message means

The full text on Linux (Firefox 157, tested) is: "Firefox is already running, but is not responding. To use Firefox, you must first close the existing Firefox process, restart your device, or use a different profile." On Windows the dialog may instead offer a Close Firefox button that ends the old process. A profile can be used by only one Firefox process at a time. On Linux the running copy holds two lock files in the profile folder. From our test with Firefox 157:

-rw-rw-r--  1 tester tester       0 Oct  8 05:36 .parentlock
lrwxrwxrwx  1 tester tester      17 Oct  8 05:36 lock -> <ip-address>:+4259

lock points to the computer and the process ID (4259) that owns the profile. Typical causes:

• Firefox is still closing, or hung, after you closed its last window.
• Another session uses the same profile: a second login, a remote desktop, an SSH session, or another computer that shares your home directory.
• A launcher or script starts a second instance with the same profile.
• Firefox cannot write to the profile folder (read-only network share, wrong owner).

A crash alone usually does not cause it on a local disk: in our test we killed Firefox with kill -9, which left the lock file behind, and Firefox still started normally and replaced it. So look for a running process before you delete anything.

Fix it on Linux

1. Check for Firefox processes: pgrep -l firefox. Our test printed 4259 firefox-bin: only the main process matches, because helpers are named Web Content, WebExtensions, RDD Process and so on. pgrep -a -f firefox matches full command lines and lists them all (12 processes in our test). For more ways to find processes, see list running processes in Linux.
2. Close them: pkill firefox. Ending the main process takes the helpers with it; in our test nothing was left after a few seconds. If Firefox ignores it, pkill -9 firefox forces it to stop without saving anything, so use it only as a last resort. More options are in how to find and kill running processes.
3. Remove the lock files if no Firefox process is running and the message still appears. The profile folder depends on how Firefox was installed:
   • Mozilla's .deb, .rpm or tarball builds and distribution packages: ~/.mozilla/firefox/. Firefox 147 added support for the XDG base directory layout, and in our test Firefox 157 created a fresh profile in ~/.config/mozilla/firefox/ instead.
   • Snap, the default on Ubuntu 24.04 and 26.04, where apt install firefox installs a transitional package for the snap: ~/snap/firefox/common/.mozilla/firefox/ (according to Mozilla).
   • Flatpak: ~/.var/app/org.mozilla.firefox/.mozilla/firefox/
   This command lists the lock files in all of them; add -delete at the end to remove them. -maxdepth 2 keeps it away from unrelated files named lock deeper inside the profile:

   find ~/.mozilla/firefox ~/.config/mozilla/firefox ~/snap/firefox/common/.mozilla/firefox -maxdepth 2 \( -name lock -o -name .parentlock \) 2>/dev/null

4. Start Firefox again.

SSH, remote X and second instances

Normally, starting Firefox a second time hands the request to the running copy, which opens a new window. A Firefox started from an SSH session with X forwarding, or from another login, cannot reach the copy on your desktop, so it finds the profile locked. In our test a second Firefox started in a separate session printed exactly the message above, and --new-instance on its own gave the same error because it still used the same profile. Either close the desktop copy, or give the second one its own profile:

mkdir -p ~/ff-remote
firefox --new-instance --profile ~/ff-remote

In our test a second Firefox with its own profile folder started without the error. firefox -P opens the Profile Manager if you prefer a named profile. Mozilla also advises removing the -no-remote option from any shortcut that uses it.

Fix it on Windows

1. Press Ctrl + Shift + Esc to open Task Manager. On the Processes tab, right-click Firefox and choose End task. From a Command Prompt, taskkill /F /IM firefox.exe does the same (/F forces the processes to end, according to Microsoft).
2. If the message stays, press Windows key + R, enter %APPDATA%\Mozilla\Firefox\Profiles\, open your profile folder and delete parent.lock. The Microsoft Store version keeps its profiles under %LOCALAPPDATA%\Packages\ instead.
3. If your profile is on a network share, open the share in File Explorer before starting Firefox, and make sure the profile folder is not set to Read-only in its Properties.

Fix it on macOS

Press Option + Command + Esc, select Firefox and click Force Quit. If the message stays, delete .parentlock in your profile folder under ~/Library/Application Support/Firefox/Profiles/.

If nothing helps

• Check ownership: Firefox must be able to create files in the profile folder. Run ls -ld ~/.mozilla/firefox/*/ (or the snap or .config path). If they belong to root, for example because Firefox was once started with sudo, give them back with sudo chown -R "$USER": ~/.mozilla (use your own profile path).
• Restart the computer. This ends hung processes in other sessions too.
• Use a new profile: create one with firefox -P and copy your bookmarks and passwords across, as Mozilla describes in its profile articles.

Official documentation: Mozilla: Firefox is already running but is not responding, Mozilla: where Firefox stores your profile, Mozilla: install Firefox on Linux.