All of your personal settings, bookmarks, and other information are kept in your Firefox profile. Firefox needs an unlocked profile to start up, and if the profile is locked, it displays the message, Firefox is already running, but is not respondingA copy of Firefox is already open. This article explains what to do if you see this message and how to prevent it from showing at startup.
If Firefox did not shut down normally when you last used it, Firefox might still be running in the background, even though it is not visible.
If you try to open it while it's running, an error message will appear with the option to click Close Firefox. If that doesn't solve the problem, restart your computer or try the following options. Restart your computer to see if the problem goes away or try the following options.
Restart your computer to see if the problem goes away or try the following options.
If you store the profile on a file share, try accessing the file share beforehand using Windows Explorer.
Firefox must also have the right to create files in the profile folder. The error can occur if you try to use a profile from a file system mounted with read-only permission (e.g. a remote Windows share which doesn't have "Allow network users to change my files" checked).
To check if the profile folder has the proper permissions:
If you are unable to unlock the profile using the above methods, use the Profile Manager to create a new profile and migrate your data from the locked profile to the new profile. For information on how to do this, see Recovering important data from an old profile.
Based on information from Profile in use (mozillaZine KB)