For a silent install use the command line option:
-ms
example: Firefox Setup 19.0.2.exe -ms
FirefoxSetup20.0.exe -ms
i've deleted the spaces in the Filename.
For including extensions in your package be mindful of whether a default per-user installation is required which generates user specific files in user profile or whether extension should be available to all users.
Global extension installation requires identifying the GUID reference in the install.rdf (This is contained in the xpi file)
http://kb.mozillazine.org/Installing_extensions
<em:id>{9D6218B8-03C7-4b91-AA43-680B305DD35C}
Rename the xpi file to use the GUID reference and place it application directory below. E.g. {9D6218B8-03C7-4b91-AA43-680B305DD35C}.xpi
C:\Program Files\Mozilla\Extensions
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