Silent install instructions can be found on the Mozilla Wiki.
If you wish to use the default settings for Firefox (including the install directory and user options), you can just use the silent option (must be uppercase):
In order to customize the installation to your environment, you need to create an INI file which will be referenced during the install. This is an example INI file:
The above example would cause Firefox to be deployed in the Program Files directory in a directory called Mozilla Firefox and a Firefox icon will be placed in the Quick Launch and Start Menu under the directory Firefox. Using this example a shortcut would NOT be placed on the Desktop.
To use the INI file, copy it onto a file share accessible from the target computers and use the following option:
Replace the given path with the path to your INI file.
If you use the INI installer option you need to ensure that this checkbox is checked so that the target computers will have sufficient permissions to read the INI file on the server.