Is it possible to disable new profile creation in Firefox? Or password protect new profile creation?
The problem is there is no point in installing say site blocking add-ons because anyone can create a new profile and access any site they want.
If you don't give your users administrator access, then it should be enough to revoke write privileges on
The location of this file may vary depending on your setup.
Note that this file is straightforward, so you can keep control of what profile is loaded as well.
Site blocking is best done at the perimeter however.