Firefox woes - "Firefox is already running" when it's not

Today on my lounge ‘browsing’ PC, Firefox stopped working. Every time I click on the shortcut to launch it, I get a pop-up message box saying:

“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.”

I resolved to find out what was going on, and how to fix it.

Needless to say, I first tried both of the things mentioned in the error message. There’s no Firefox process anywhere; and restarting the machine made no difference. Googling around wasn’t much help, the forums I found mentioning this problem just suggested ‘have you tried rebooting’ or ‘run task manager and kill the FF process.’ So no joy there. The only suggestion that seemed to be working was deleting the .mozilla folder (on linux in your home folder, in Windows in the relevant user’s Application Data).

I’m not a fan of randomly deleting folders without understanding why it was needed, so I decided to investigate further. This problem was happening on my Linux box, so I decided to strace the Firefox process and see what it was up to. On a Windows machine I would have used Filemon from SysInternals.

I ran the command: strace -o ~/ff.strace firefox and then investigated the strace file. My hunch was that one or other file lock wasn’t being relinquished properly, perhaps from an earlier crashed firefox process. I grepped through the log, looking for file opens, and eventually found this:

open(“/home/matthew/.mozilla/firefox/2z7l4uii.default/.parentlock”, O_WRONLY|O_CREAT|O_TRUNC, 0666) = 4

Bingo! I checked; and even with no firefox process running, this file existed. It was an empty lock file, so I deleted it. That did the trick — now FireFox runs again!

So in summary; if you have this problem, check your .mozilla file (or the Windows equivalent) for any ‘lock’ files — quit any mozilla applications, then delete the lock files and try again. That should fix the problem!