Yesterday I upgraded from Moutain Lion to El Capitan on my MacBook. Previously Thunderbird worked fine. Now it will not launch and I get an error message that "A copy of Thunderbird is already open. Only one copy of Thunderbird can be open at a time." Unfortunately there is no other copies of Thunderbird running. If I look in the "Force Quit" window I don't see Thunderbird. I tried deleting and re-installing Thunderbird with latest version. I still get the same message. Firefox works fine. Can anyone help? Thanks!