Here is how you do it. Quote from Barry`s blog by Iguleder:
It means that if you remove your /root/.mozilla content, the extension is not removed. Generic installation. It is nice trick for puppy builder...it gives possibility to push extensions preinstalled to the Firefox. I have flashblock, adblock and fireftp preinstalled this way.....Barry, I found a way to pre-install extensions for Firefox and Seamonkey without having to create a profile first!
It's really simple - I found out that xpi extensions are zip archives. Just unzip an extensions, open install.rdf and get its ID, which is a GUID.
In the case of TorButton, it's "{e0204bd5-9d31-402b-a99d-a6aa8ffebdca}".
Here's how to pre-install TorButton for all users, if so (assuming Firefox is located in /usr/local/apps/firefox):
install -m644 -D torbutton-current.xpi $INSTALL_DIR/usr/local/apps/firefox/extensions/\{e0204bd5-9d31-402b-a99d-a6aa8ffebdca\}.xpi