so i've just gone through this process, and you can add the extensions in as 'global'.
effectively, when you get to the 'Run' part of your sequenced apps, run firefox. Then shut down firefox, and delete the Mozilla folder out of %APPDATA%. Then, you need to download the XPI's of the extension you want installed to the install path for your sequenced app, mine as an example is Q:\Firefox\Extensions. Once you've got your XPI's in this location, run the following:
Q:\Firefox\firefox.exe -install-global-extension "<path-to-extension>\extname.xpi"
This will install the extension as a global extension available by default for all users.