How do I prevent Firefox from installing new addons?

This is for my niece account. Firefox 3.6 on Vista Home Premium.
I would like to prevent her from installing new addons (possibly, but non necessarily allowing her to update the existing ones).

I have already installed Procon Latte that essentially does nothing :-(
I selected the following options in Latte:

Hide from Tools Menu
this should remove the the ProCon shortcut from Firefox's Tools menu
YEP, BUT it's still visible in the addons list

Lock the Add-ons list
this should lock the Add-ons window, to prevent ProCon (or any other extension) from being altered
IT DOES NOT LOCK ANYTHING BUT Latte, I can still update other extensions and add new ones.

Lock 'about:config'
This prevents the config page from coming up

Browsing Firefox KB I found the xpinstall entry in the config file. I set this to false and yes, it does not allow new extensions to be added, except that IT ASKS TO "CLICK HERE TO ENABLE IT", and lo and behold, the config file gets updated - in the face of Procon Latte - and new extensions can be added.

Anyone can help?
It would be nice in xpinstall.whitelist allowed for extension names to be added, but it seems bounded to websites. She could still download the xpi and then install it from her desktop, I suppose.

 

Edit host file and route every page you don't want to be accessible to localhost.

 

Thanks sama98b, but that is not a feasible solution. I don't know in advace which pages hold the trillion possible addons she could find. I need to block the installation of any new extension.
It's kind of funny that the xpiinstall option can be overridden with a click. Security my ---

 

If it's security you're worried about, then might I suggest setting up a sandbox service (sandboxie)? It won't actually prevent an addon from being installed (permanently), but at least you'd be able to set it up to wipe out anything she'd done during the session. It would also prevent her from bookmarking and using cookies for credentials however, so it all depends.

 

Thanks Akira, I hadn't thought of setting up a sand box. It's a bit too much, though but I might use this approach with my other younger nieces.

I believe I've fond the add-on I was looking for
https://addons.mozilla.org/en-US/firefox/addon/public-fox/
It can set a master password for accessing the addons box. I haven't checked if it really prevents addons from being installed but it looks promising.

 

Why don't you just create a separate account for your nieces so whatever they do doesn't affect your own work?

 

The older one already has her own computer with a limited account.
My sister wants her to use the laptop responsibly, but since she already bypassed Parental Control's time limitation by changing the BIOS clock, now she's - how to say - using it under parole.
And most of the lockable stuff has been locked up.

She will eventually find a way around the blocks, I know.
That's part of the 'game'.
My job is to make that bypassing a bit harder.

As for the younger ones, they also have separated accounts, with all of the Internet blocked. But they learned how to stress their parents to get their account's password out of desperation. I guess there's no security addon that could prevent that. :-(