How to Prevent Firefox Add-Ons from Being Installed?

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.