After the removal of the '-install-global-extension' option, deploying firefox addons has become uniquely painful. Here's how I've successfully done it by hand.

rem (copy addon folder)
xcopy /siy {0C46772F-D59F-4b47-B2FD-F37E3F979437} "c:\program files (x86)\mozilla firefox\extensions\{0C46772F-D59F-4b47-B2FD-F37E3F979437}"

reg add HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Mozilla\Firefox\Extensions /f /v {0C46772F-D59F-4b47-B2FD-F37E3F979437} /d "C:\Program Files (x86)\Mozilla Firefox\extensions\{0C46772F-D59F-4b47-B2FD-F37E3F979437}"

rem didn't work without this file
copy /y extensions.sqlite C:\Users\Default\AppData\Roaming\Mozilla\Firefox\Profiles\20ohj5r0.default

rem bits are 1 + 2 + 4 + 8
rem 4 & 8 bits are for all users, you might have to use 0 if you copy the addon to the user's profile
echo user_pref("extensions.autoDisableScopes", 3); >> C:\Users\Default\AppData\Roaming\Mozilla\Firefox\Profiles\31pij5r0.default\prefs.js

rem bonus: hide addon toolbar
copy /y localstore.rdf C:\Users\Default\AppData\Roaming\Mozilla\Firefox\Profiles\31pij5r0.default