I actually think I have answered my own question (sort of). So I narrowed the line of code that was causing the problems down to this one:

Set objFile = fso.CreateTextFile(temppath)

What I am trying to accomplish with this is to create a dummy text file in the profile folder that signifies that the script does not need to run again. Unfortunately, it doesn't work (doesn't create the text file) and doesn't throw any errors either. Also, after running this line of code, the path:

ExpandPath("%AppData%\Thinstall\Mozilla Firefox\#25AppData#25\Mozilla\Firefox\Profiles")

is no longer valid. Any insight on this strange behavior?

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