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    This solution is still valid after almost 4 years (just tested with Firefox 53 on Ubuntu 17.04). Thanks! Commented Apr 26, 2017 at 14:26
  • Should be the answer. I had the problem when another user had logged into firefox, so I couldnt kill the PID.. I had to remove these files.. Commented Aug 7, 2017 at 3:00
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    Blind one-liner: rm ~/.mozilla/firefox/*.default/{.parentlock,lock} Commented Jan 9, 2020 at 22:04
  • 1
    for me the directory was named *-default rather than *.default Commented Feb 28, 2020 at 16:45
  • 1
    Or even more nuclear (but potentially necessary if you've created a non-default profile): rm ~/.mozilla/firefox/*/{.parentlock,lock} Commented Aug 4, 2021 at 17:35