If you get this message, it's a clear sign that your profile has been locked, and that, subsequently, it wasn't unlocked properly. Firefox locks your active profile whenever it runs, in order to prevent other applications altering it. This is intended to stop other applications from altering the profile; however, living, breathing people always seem to find a way to do so! If this happens, you're left with an unwanted lock that must be cleaned up by hand, using the following steps:
1.
Kill any Firefox processes that are running, using the tip provided above. Mac OS X users should open Force Quit Application using Command Option-Esc, select Firefox, and click the Force Quit button. Linux users should kill the Firefox process by locating its process ID, and sending it a signal. This command should do it: pkill firefox.
2. Delete the lock file named "parent.lock" in Windows, ".parentlock" on Mac OS X, or "lock" in your Firefox profile directory if you're running Linux.
Firefox keeps a lock file in your profile directory whenever it acquires a lock on that directory. Sometimes, when Firefox is not shut down properly, the lock file is left in place, causing Firefox to think the profile is locked. Deleting the lock file solves the problem. Once that's done, Firefox should start normally.