While using a Ubuntu linux machine, this can happen for a lot of people. This is the solution.
Error Message: "Firefox is already running but is not responding ..."
Many packages (like firefox) create lock files in your account to prevent database corruption with multiple copies of the application. Look in your account HOME for recent lock files and remove them if you're not logged in to other machines.
The solution is easy:
1. find the .parentlock file under user's directory
find -name "*.parentlock"
2. delete those files.
3. possible problems - the bookmark and history does not work properly. What I did is deleting the entire firefox directory where .parentlock file resides. Of course, I lost my history and bookmark. But that's OK, they are not important for me.
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