Find firefox log file

Because it can easily fill up your logs, the default is to not log. Add a jump to the LOG target, which will log to the kernel log (which you can see with dmesg or at wherever syslog is configured to write that for your distro). In your LOG-target rule, you can set --log-level and --log-prefix to help organize the messages and keep them separate from other kernel messages.

LOG is a "non-terminating target", so rule traversal will continue on to the next rule — you can basically add logging right above your existing rules without affecting them.

From http://kb.mozillazine.org/Profile_in_use - Check for:

1) A file called .parentlock
-AND-
2) A symlink called lock
...in ~/.mozilla/firefox/random string of alphanumeric characters.default/

The former is present while Firefox is running to prevent more than one different instance from trying to simultaneously write to the same set of profile files, and if Firefox is not closed cleanly, neither may be removed.

Error message received in this case should be:

From anywhere in the filesystem (or removing this if pwd reports ~/.mozilla/firefox/random string of alphanumeric characters.default), replacing random string of alphanumeric characters with the FF profile folder on your computer, do rm ~/.mozilla/firefox/random string of alphanumeric characters.default/.parentlock ~/.mozilla/firefox/random string of alphanumeric characters.default/lock` (assuming the first sentence pans out) to remove them, if there isn't a Firefox process running.

More less-authoritative information here.