Note: Starting in Firefox 34 on Windows (Vista and above), the 'Firefox is already running' dialog includes a 'Close Firefox' button (shown here) that, when clicked, will dismiss the dialog, end the Firefox process(es) and, after a few seconds, will automatically restart Firefox. The old Firefox process must be closed to open a new window. 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 running copy of Firefox will quit in order to open this one. (On Mac OS) A copy of Firefox is already open.Only one copy of Firefox can be open at a time.'

A 'Close Firefox', 'Close Thunderbird', or (starting in SeaMonkey 2) 'Close SeaMonkey' dialog box appears with one of these messages (using Firefox as an example) :.You see an error that your SeaMonkey or Firefox 'profile cannot be loaded.You start the application and it won't run, but it doesn't display any error messages.You receive a message that the selected profile is already 'in use' while using the Profile Manager.

This article explains how to regain access to a locked or unavailable profile.

Lock files in windows