"Thunderbird is already running, but not responding. To open a new window you must close the existing thunderbird process, or restart your system"
This is a stupid message i know. I searched through Google and Mozilla forums and didn't really find any straight forward answers so I am hoping to give you that here.
Most people will suggest that Thunderbird wasn't closed properly and has left a lock file behind that causes this error but i never found any "lock" file (sometimes called "parent.lock" and has restarted my computer improperly with Thunderbird on many times but never the less here we go!
This scenario solution is based on people who have their profile folder for Thunderbird moved to a non-default location (to back up there mail or avoid defrag or whatever) or has moved the folders of the default folder. If not try this solution anyway.
- Open C:\Documents and Settings\USERNAME\Application Data\Thunderbird\profiles.ini
- Change the value after "Path=" to "Path= C:\CURRENT FOLDER LOCATION" (no")
- Save file
- Open Thunderbird or restart and then open Thunderbird.
Hope I have made someone's troubles go away ;D
EDIT: WINDOWS 7'S FOLDER IS LOCATED AT: