There are related entries in the Mozilla database like "https://support.mozilla.org/de/kb/firefox-wird-bereits-ausgefuehrt-reagiert-aber-nicht?redirectlocale=en-US&redirectslug=Firefox+is+already+running+but+is+not+responding" in varous languages but they do not really tackle the cause of the headache. My own observation: 1. Go to any web-site offering a mailto-tag. Use it. A window opens. You may write your mail. 2, Now you have the idea you want to copy some text from an old mail. Go to desk-top and start Thunderbird. It will complain another instance is already running but not responding which is kind of nonsense in the eyes of a user because the user is seeing a window which does not offer any evident way to go to some other part of Thunderbird. 3. There is a way hidden in the "Extras" menu which works because this request is processed in the same user theread as any other user request. It then opens a new window with its own thread and everything works fine. It seems to me there is a serious desing weakness. You could provide some more obvious navigational option to open a second window, but much better you make the program itself more intelligent so it handles collisions behind the scenes without bothering the user. Basic idea: If it recognises a locked profile it should contact the instance owning the profile and pass the user's request to the instance owning the profile. I know, this is not a trivial request. Probably you need to split the complete program in two parts. Lets call them "The Coordinator" and "The Slave" where the Coordinator is the owner of the profile. He shares the profile in an orderly manner with the slaves doing the real work and listens on a queue for requests from other coordinator running foul on locked profiles. It seems this problem is haunting the complete family, not just Thunderbird. Maybe you should have some framework as a basis for the complete family and fix the problem once for all the members. Kind Regards Wolfpadi