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!
I am getting this since my upgrade to 21.04 (Ubuntu) and only if I want to open links from Thunderbird in FF by clicking on them. Like an “online version” of the mail, a link to an article or whatever. Unfortunately deleting lock and parentlock, using a different, new profile, changing standard browser, restart and change it back (restart again) didn’t help at all. Also, mailto-links in Firefox doesn’t open in Thunderbird with a similar error message. This is so annoying.
Thank you very much for this solution. This error was driving me crazy for a week now. Cheers
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.