The letter of the drive

So, this one is funny.
I create a new virtual machine with Windows 10 x64 updated (1709). Then I download the FirefoxPortable_57.0.4_English.paf.exe file and install it right on the desktop.
I enter that folder and create a new text file, rename it as aa.bat. Right click, Edit. And add these four lines:

start C:\Users\kanu\Desktop\FirefoxPortable\FirefoxPortable.exe
pause
start c:\Users\kanu\Desktop\FirefoxPortable\FirefoxPortable.exe
pause

Everything is the same except the c that is capitalized in the first one.
If I run it with a Firefox instance already opened, one of them won't start and reply with an error (the usual "another instance of Firefox"...). I've found out that on some computers the capitalized gets the error, in others -as the virtual machine- it's the lowercase.
That's quite a mystery.