Danny – The Firefox silent installation parameters have changed since a couple years ago. According to https://wiki.mozilla.org/Installer:Command_Line_Arguments you now have to download the full installer from http://www.mozilla.org/firefox/all/ and then execute the installer using the -ms parameter instead of /S.
I just tested deploying this with BatchPatch and it worked without any issues.
In your case since you executed the installer using /s, the exe is now running (hidden) and will never close on its own. You will need to manually kill the installer exe on the target host(s) to end it. Then proceed with the installation according to the notes above, using the -ms parameter. Just make sure that you have downloaded the full, offline installer, otherwise you’ll still have problems even when using the correct -ms switch.
I hope this helps.
-Doug