How to fix the error 'Firefox is already running, but is not responding' error in Linux machines?

I was trying to open Firefox browser in my Linux box and faced this issue:
"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"

To fix this issue, I tried looking for Firefox process and kill it, but couldn't find the process id.
>ps -ef | grep -i Firefox

finally, I found the following solution:
when we start an instance of Firefox, a lock will be created in the Firefox profile directory to prevent multiple instances of firefox. This situation can arise when system is shutdown abruptly when firefox is running or some other issue with in Firefox. 

If we clear these locks, we can start Firefox without issues:
>cd ~/.mozilla/firefox/xxxx.default
>rm  lock .parentlock

in first command, replace xxxx with the actual folder name in your system.

Now try to open Firefox again.
>firefox &

Hope this helped in resolving your issue.