I have seen this error when my Mac has auto rebooted for some reason. Firefox does not show up as a running process and can’t be found using the Force Quit… feature so what to do? It’s time to drop into a terminal session to go looking for a rogue child process that is keeping Firefox from opening.

(⎈ default)➜  ~ ps -ax | grep fire
1820 ??       312:27.66 /Applications/Firefox.app/Contents/MacOS/firefox

There is the culprit and it needs to be killed

(⎈ default)➜  ~ kill -9 1820

Firefox should now launch without an issue. If for some reason that doesn’t get it done there are two alternative things to try:

delete the .parentlock file
delete an unknown identity

To delete an unknown identity run the following command at a terminal session

(⎈ default)➜  ~ /Applications/Firefox.app/Contents/MacOS/firefox-bin -P

You should see a popup window that displays the identities known to Firefox. Delete the one(s) you didn’t create and restart Firefox.

On the machine you want to remotely connect to:

Go to System Preferences.
Click on Sharing folder.
If necessary, click the lock at bottom left and enter your password to make changes.
Put a check mark next to Screen Sharing on the left.
OPTIONAL: Click the Computer Settings… button on the right.
Put a check mark next to “VNC viewers may control screen with password:” and set a password to use for screen sharing.
Click OK.
To secure logins to only your account, make sure it’s set to “Allow access for: Only these users.” Click the plus button at the bottom of the list.
Select your name (or the name of the user account you use) from the list and click the Select button. If you would like to enable file sharing as well, put a check mark next to File Sharing on the left.

On the machine you’ll be connecting from:

Go to Finder.
Select Connect to Server from the Go menu.
In the Server Address field type in the IP address of your remote computer preceded by “vnc:”.
Click the Connect button and it will open the remote desktop in a new window. You will have control over the keyboard and mouse as if you were sitting down at that computer.
When you’re done, close that window.
To access files and folders on the remote computer, select Go to Folder from the Go menu.
Type in the IP address of the computer you wish to connect to preceded with “afp:” as it appeared in File Sharing above.
Click the Go button and the folders you have access to on the remote computer will open in a new Finder window.
When yo u’re done, close that window.

A copy of Firefox is already open. Only one copy of Firefox can be open at a time.

How to setup / use Mac to Mac remote desktop connection