"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"

"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". There are plenty of discussions regarding this problem. But none of them address the problem context as I am encountering. Vista SP2 32Bits Firefox 5.0 on it. Thunderbird 3.1.1 1. Firefox is running and open. 2. User opens in Thunderbird some mail message including an url 3. User follows the url by clicking on it 4. Error message comes up similar to following "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" 5. In order to open the web page assigned to link from mail message open in Thunderbird the user needs first to close FX manually, then follow the url. Not always reproducible. How to solve this problem?