Recently, after terminating Thunderbird (60.6.1) on Windows 10 (v1809), it will not start again, reporting: "Thunderbird is already running, but is not responding. The old Thunderbird process must be closed to open a new window". W10 Resource Monitor shows that process components remain in memory, long after thunderbird.exe has terminated. I also have a similar problem with Firefox (v66.0.3), showing the same failure to eliminate Forefox components from memory after termination, and the same failure to restart. The only way I have found to fix the problem is to reboot. Ugh!