I made this script about a month ago, I had to tweak it to also run on vista more recently, for compatibility. I have had no problems from it.
I figure it may help with the firefox launcher or at least to come up with some ideas. it is a batch script not NSIS code.
it does not care about the current directory, it does need to be on the same USB drive though.
It backs up the host computer to a folder on the flash drive, "\PortableApps\FirefoxPortable\Data\Flash Cookies\Host Cookies"
then it deletes the host's directory, "%APPDATA%\Macromedia\Flash Player"
& copies your cookies, "\PortableApps\FirefoxPortable\Data\Flash Cookies\My Cookies"
then runs firefox portable
(firefoxportable.ini needs to be set to keep firefoxportable.exe open)
copys "%APPDATA%\Macromedia\Flash Player" to "\PortableApps\FirefoxPortable\Data\Flash Cookies\My Cookies" & restores the host cookies. deleting your cookies from the computer in the process