Preinstall Firefox Extensions for All Users?
 

I'm sure I'm not the first to think of this, but I can't find an answer anywhere online. Perhaps my search terms aren't specific enough or something.

I want to install certain firefox extensions such that if I add any new user those extensions will be available to them. Preferably, it should also work out that if I add a new extension for all users that existing users will have it too. /etc/skel won't work because it would waste a lot of disk space for each user to have their own copies, and it wouldn't update them.

I've found where plugins such as java and flash should go: /usr/lib/mozilla/plugins

Where can I throw an extension that I want made globally available?
It would be nice to do this with themes as well.