Since FileToCopy contains a full file name, when you concatenate it with ProfileFolder you get an invalid file name, like this:
C:\Documents and Settings\username\Application Data\Mozilla\Firefox\Profiles\mlreq6kv.default\D:\unproxy\user.js
Change this line to the one below, and your script should work fine. (Note: the trailing path separator at the end of ProfileFolder is required to indicate that the profile folder, e.g. mlreq6kv.default, is indeed a folder and not a file.)
After a lot of messing around, I got it (actually found it in the InstallShield manual, and not Google, go figure :))!
This line of VBScript does the trick
disk1Path = Session.Property("SETUPEXEDIR")
The line above points to where ever the setup exe file was, so from there it's trivial to run any exe included with your install media.
Other useful ones I found, which I'll past here for reference are
'points to app data\downloaded install directory
'where the software wants to install to on the users system
Not sure why it's so hard to find a good reference on MSI Standard properties (even just a list). The closest I found was this, but not all of them work (and not specifically for InstallShield at all). If anyone finds a good link with documentation about MIS Standard properties and their description please add a link here, so no one has to waste as much time on this as I did :).