New
#31
The behavior seen here is not a problem and there is no culprit. It is only some program doing exactly what it was designed to do. As the temporary file was created by Firefox it seems reasonable that Firefox itself would delete it. This is recommended behavior and most people would expect it. I certainly would. Not doing so would be seen by many as a security or privacy risk, and indeed it is. I would not expect any sort of setting to disable this.
Temporary files are themselves a necessary evil and have some well known security issues. They really should be securely disposed of as quickly as possible after they are no longer needed. Unfortunately the user isn't in the best position to judge when that is. Windows delays deleting them because developers sometimes have different expectations of what "temporary" means and expect them to be available well beyond when they were created. Not recommended behavior but it happens.
You really have no assurance that temporary files will survive beyond when they are no longer needed by the process that created them. It would be best to follow the advice of Barman58 and use other methods to obtain the information needed.