Automating Mozilla installation
This is pretty funny, because you don’t really need a setup program.
You can download the zip file instead of the setup program. After unzipping it, it will not become the default browser, but when you will run it for the first time, you will prompted for this, and the file type and URI protocol associations will be created.
According to Unattended project on SourceForge, Mozilla and Netscape installers used to have a switch -ma that disables the GUI and -ira, that disables the automatic launch of the application after the installation. This might still work, but I’m not sure. This way, you cannot set anymore the installation directory.