I receive an error, "Firefox is already running, but is not responding" when trying to use Firefox.

Firefox profiles are stored in the user's home directory. MSI systems share home directories across various machines. An instance of Firefox requires exclusive control over these profile resources across various machines. If you try to run Firefox in two places, such as a lab machine and a remote portal, you will receive this error. The error will also appear if you try to run Firefox on both lab Windows machines and the Windows server. 

To solve this problem, one of three approaches can be taken: first, close the browser on the other machine; second, create a new profile; or third, clear out the lock files that prevent Firefox from opening. 

Solution #1 (Easy)

Close the browser on the other machine.  This will resolve the problem in most cases.

Solution #2 (Intermediate)

Use Firefox's profile management, to create a new profile and run many instances of Firefox simultaneously. To do so,

  • On Windows machines, go to Start > Run, and type in "firefox -P"
  • ​On Linux machines, open a terminal and enter "firefox -P"

Here you can create, rename, and delete Firefox profiles. If you'd like to choose your profile each time you start up Firefox, uncheck "Don't ask at startup." You can find more details in the Firefox's Managing Profiles page.

Note that information sharing between profiles is limited, so your bookmarks and form data will not be available. 

Solution #3 (Advanced)

Delete the lock files that prevent Firefox from launching. The following shell script will find and remove any lock files that prevent firefox from executing. Save this script with filename cleanFirefox.sh: 

# Use this script to clean locks inside the .mozilla directory that 
# prevent firefox from running on multiple machines that share an 
# NFS system. If you open firefox and get a window claiming, 
# "another instance of this application is already running [...]" 
# running this script will enable you to open the application on another
# host.

files=`find ~/.mozilla -name "*lock"`
for file in `echo $files`
    echo "removing $file..."
    rm "$file"
To execute the script:
  • open a terminal
  • cd to the directory where you saved the script
  • run: sh cleanFirefox.sh


Additional Questions