After I manually reboot my Ubuntu machine, when I try running Firefox, sometimes I get the following error:
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.
The solution is to delete a hidden mysterious file called .parentlock. This file is typically located under your home directory in the following location.
cd ~/.mozilla/firefox/
You should see a folder with some random name (yours will differ), followed by .default, for example: 06agjsjz.default
Go into that folder:
cd 06agjsjz.default
There should be a file called .parentlock (it’s hidden so type ls -a to see it). Delete it:
rm .parentlock
Start Firefox again and it should work!