It's possible to register non-profile extensions in the registry, specifically so software installed on the system can be shared across user accounts, and be in

It's possible to register non-profile extensions in the registry, specifically so software installed on the system can be shared across user accounts, and be installed/uninstalled orthogonally to Firefox. The downside, of course, is that Firefox doesn't know how to uninstall these extensions. You can of course still disable them, which has almost exactly the same effect.