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.
This is fixed in Thunderbird Portable 91.4.1. The Thunderbird directory was lowercase in the launcher define code, causing the CityHash to be different.
Sometimes, the impossible can become possible, if you're awesome!