How-To

How to Fix the Firefox is Already Running Error

By Brian Burgess

Updated November 22, 2023

Mozilla Firefox is a great web browser with many features unavailable in other browsers. But occasionally, you may need to fix the “Firefox is already running” error. The error message typically also states the application is not responding, the old Firefox process must be closed, and your system needs a restart.

This was a common error several years ago on older systems. It occurs when Firefox is closed and still running in the background. It can happen when Firefox is in the process of closing or hasn’t shut down properly. Also, it could be an issue with your profile in certain instances.

With tabbed browsing the norm and an updated Firefox design, it’s now rare to see this error. But you may still encounter it on modern computers.

There are a few ways of resolving the problem to get past the error and get on with your work. We’ll show you how to do it in this guide.

Fix the Firefox is Already Running Error

The “Firefox is already running” error has plagued users for years. Contrary to what the message implies, there are other ways to get rid of the error that doesn’t require you to restart your system. However, if all else fails, a restart is the nuclear option of choice. But we want to avoid that, so try these options first.

Close the Firefox Process

1. One way to close Firefox is by simply clicking the Close button (X), but if that doesn’t work, use Task Manager, as shown in the following steps.
2. Right-click an empty area of the taskbar and select Task Manager from the menu that appears.
3. Select the Processes tab, find and right-click Firefox, and select End task from the menu.
4. Wait a moment after you close out of the Firefox process and relaunch the browser. You should no longer experience the error.

Remove Firefox Profile Lock

If you continue to have an issue with the error appearing, there’s the possibility that your profile is locked. The browser is designed to ensure only one profile is used during a session.

Firefox places lock files in the profile directory but could leave it behind if Firefox crashes or isn’t properly shut down. So, to remedy the problem, you can manually delete the profile lock file. Note that you need to completely end the Firefox process via Task Manager before starting these steps to prevent profile corruption.

1. Press Windows key + R to launch the Run dialog box.
2. Type in the following path and click OK or press Enter.

   %appdata%\Mozilla\Firefox\Profiles

   

3. Open the .default folder — it will have a randomly generated name on each system.
4. Once the .default folder opens, locate and right-click the parent.lock file and select Delete from the context menu that appears.

Wait a moment after deleting the file, close File Explorer, and relaunch Firefox, and it should run without any problems with the error.

Note: On macOS, you need to delete the .parentlock file in the profile directory, and on Linux, you need to delete the .parentlock and lock files to fix the error occurring because of Firefox’s profile lock.

Restart as Last Resort

No one likes to go through the hassle of restarting their computer, but if you’ve exhausted the options above to fix the “Firefox is already running” error, you may need to do it.

Before restarting Windows, ensure that you save your work, close any open apps or processes, and restart your Windows PC.

Of course, if you are on a Mac, you can restart macOS to resolve the error and get Firefox working correctly again.

1. Click the Apple logo in the top left of your display.
   
2. Click Restart, and your Mac will reboot.

Fixing the Firefox is Already Running Error

Getting this error was commonplace just a few years ago. But with tabbed browsing and the new design of the Firefox browser, it’s less common to encounter the “Firefox is Already Running” error message. However, that’s not to say it won’t appear, especially on older systems running legacy browser versions.

The good news is that if you encounter an error, you have a plan of attack to remedy the issue. Try the first set of options to fix the error and only use the restart option as a last resort.