How to prevent Firefox add-ons from being installed?
By John Peck •
Firefox asks before installing add-ons but how can I make it so the user is not allowed to install add-ons?
21 Answer
Add-ons are not installed system-wide but by each user just for herself. And they are stored within their home directory.
So there is no way (apart from making the profile directory read-only) to prevent a user from installing them.
In case you want to experiment with the profile: visit about:profile within Firefox to find out where your profile is stored. But keep in mind (as Dan stated in a comment) that nothing can prevent a user to create and use a new profile for her browser.