So if you are like me, I had trouble figuring out how to do a silent install of Firefox. This is how you do it:
Go to Run & type %temp%
Run the setup file that was downloaded from firefox.com, wait for the files to extract.
Once the files are extracted, look for a folder at the end of the list.
In the folder there should be a file named setup.exe, in its properties you will see that it is the Firefox installer.
Copy the contents of the folder to your scripting directory.
You can then run setup.exe -ms to run the silent install.
Have a great day!