Firefox won't launch

When I try to launch Firefox, I get an error message that Firefox is running but not responding and buttons to Close Firefox or Cancel. Closing Firefox results in a pause, then the error message reappearing. When I monitor in Task Manager, I see 2 Firefox processes. One closes when I click Close Firefox, then reappears before the error message reappears. If I close both processes before launching Firefox, two new processes appear with the error message. I can't open Firefox in troubleshooting mode. I have uninstalled and reinstalled, still can't launch.