Iy is not the action of closing firefox, it is the clearing of flash cookies, so it will happen if History or cookies are cleared. It is a well known issue ( Bug 633427 ). Unfortunately it is likely to open an instance for each plugin you have, '''that includes plugins that are disabled'''. A bug has been filed and allocated to a developer, hopefully it will be fixed soon, maybe Firefox 9 or 10. I have not tried but possibly creating a new profile without unecessary plugins may help, otherwise it is a case of uninstall unused plugins, or wait for the bug fix.