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How to fix “Firefox is already running” error

So you log onto your Linux machine, and run Firefox. But you can’t, and you get the following error message:

Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system.

Pretty annoying right? Here’s how to solve the problem:

Open up terminal, and find your profile.

~/.mozilla/firefox/(profile name)/

For example:
cd ~/.mozilla/firefox/(profile name)

Remove the following two lock files:

lock
.parentlock

For example:

rm lock
rm .parentlock


You should now be able to run Firefox!