Sometimes when you try to start Firefox, it warns you that Firefox is already running. The message looks like this: 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. Usually, you can just kill the firefox process to solve this problem. For example, on Linux the command “ps auxwww | grep firefox” will find the process number and then “kill [processnumber]” will work fine. But sometimes things are more horked than usual. That happened to me today, surprise surprise 🙂 Here’s how to fix the deeper problem: Step 1. Find your profile. This page tells you how to find the location of your Firefox profile. Under Linux (e.g. Ubuntu), it will be at ~/.mozilla/firefox/[Profile name]/ . Step 2. Remove the lock files. This page tells you what the lock files are for Firefox on Windows/Linux/Mac. Under Unix/Linux, you’ll need to remove two files “lock” and “.parentlock” .