Thunderbird Portable 91.4.0 leaves an empty file named UpdateLock-XXXXXXXXXXXXXXXXX in ProgramData after being closed. This is due to the way Thunderbird calculates the CityHash of the path it is running from.
I felt it was more important to update Thunderbird than to have it perfect. I spent a few hours poking around the 2GB of Thunderbird source to get it to match. I'm likely just missing a small detail. I'll be working on this to get it sorted in the next release.