"Thunderbird is already running, but not responding. To open a new window you must close the existing thunderbird process, or restart your system."
Which, of course, you have already tried—to no avail. If Mozilla Thunderbird refuses to start and complains about another instance or a profile in use instead, the cause may be a stale profile lock left from a crashing instance of Mozilla Thunderbird, for example.
You can remove the file that locks your profile and try to get Mozilla Thunderbird starting again.
Fix Mozilla Thunderbird Not Starting
To help Mozilla Thunderbird start when it displays "Thunderbird is already running, but not responding" or opens the profile manager stating your profile is "in use":
- Make sure no Mozilla Thunderbird process is running:
- In Windows, kill all "thunderbird" processes in Task Manager.
- Using Mac OS X, force quit all "thunderbird" processes in Activity Monitor.
- Under Unix, use "killall -9 thunderbird" on a command prompt.
- Open your Mozilla Thunderbird profile folder.
- Delete the "parent.lock" file.
- Mac OS X:
- Open Terminal.
- Type "cd " at the prompt.
- Grab the icon in the Finder's folder title and drag it to the Terminal window, dropping it there; the line will now read something like "cd /Users/me/Library/Thunderbird/Profiles/xxxxxxxx.default".
- Hit Enter.
- Type "rm -f .parentlock".
- Hit Enter again.
- Delete "parentlock" and "lock".
- Start Mozilla Thunderbird.