Can't delete parent.lock file and Firefox won't start

scenario: This has happened to me about 5 times. Windows 10 personal pc, Firefox not responding error. I close Firefox. When I try to restart, it says it can't restart, Firefox is still running. Try to close Firefox process via Task Manager, and get "Access not authorized" error. Search the internet for solutions, various sources including Firefox help say to delete "parent.lock" file in the Firefox Profile. Try to do this, but get an error saying the file can't be deleted because it's still in use (i.e Firefox is still running). Delete Firefox, download and reinstall, reboot, nothing helps, Firefox is still running error remains. To sum up: 1. Firefox says it's running but it's not. 2. Can't access Firefox profile manager to create a new profile. 3. Can't delete parent.lock file. 4. Therefore Firefox is not available and is caught in a loop. Are there any suggestions for next steps? Thanks