[Same experience with newest versions of Portable Firefox]
Windows 10 Pro 1909 18363.1139
Since updating to 78.4.0 -- or perhaps since a previous but very recent update -- an 'unkillable' Thunderbird background process is orphaned when exiting the program. Task Manager lists the process as 'Thunderbird.' There's no response to an 'End task' command, the process just sits there. Using the Sysinternals Process Explorer, I get a message "Error terminating process: Access is denied." Trying to restart Thunderbird fails with "Another instance of Thunderbird is already running...." These problems have persisted after updating to 78.4.1 -- just now I had tried to exit TB to update to 78.4.2, but the installer doesn't run either, since it detects the residual process.