The -ms switch doesn't seem to work with 25.0.1, however, you can find MSI installers here:
Run msiexec.exe /qn /norestart /I FirefoxSetup25.0.1.msi
Using the MSI for 26.0 as found here: http://www.frontmotion.com/Firefox/download_firefox.htm
the command line: msiexec -msi /i "Firefox-26.0-en-US.msi" /qn results in a silent install.
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