User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.2.15) Gecko/20110303 Firefox/3.6.15 GTB7.1 ( .NET CLR 3.5.30729; .NET4.0E) Build Identifier: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.2.14) Gecko/20110221 Thunderbird/3.1.8 There are several open bug reports concerning this message, and the chief reasons for it are easy to diagnose and fix: 1. Non-existent profile named in profile.ini (e.g. b/c of a typo or folder move). 2. Read-only profile. 3. Lock file (parent.lock on Windows systems)left over from previous program execution. 4. And, finally, the problem this message is actually designed to cover: Thunderbird is already running and is "stuck." Anecdotally, based on my research, this almost never happens. The solution seems simple to me: create a more helpful error message, instead of using the current message that usually steers the user in the wrong direction. One fix might be ti simply refer the user here: https://support.mozilla.com/mai/kb/Firefox%20is%20already%20running%20but%20is%20not%20responding Thank you for all of your hard work on Thunderbird. Truly an awesome product! Reproducible: Always Steps to Reproduce: 1. One way: edit profile.ini and munge the path to your profile. Voila. 2. Run Thunderbird. Actual Results: Message box: "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" Expected Results: See above -- error message provides erroneous instructions for this error.

good info. definitely a duplicate (not in thunderbird bugs)