Probably not, there isn't anything like that built into Firefox. Anything a user can do himself would involve locking the \extensions\ folder or emptying the Allowed Websites - Addons Installation list, but that would only prevent "online" installation of extensions. The worst "offenders" are programs that the user installs which create registry entries and "install" extensions that only the blocklist.xml mechanism can deal with.

The only possible way that I can think of is to add the 20,000 plus extensions that you don't want to the blocklist.xml file, and then lock it so that Mozilla can't update it.

Thanks for the answer Edmeister,

I'm thinking about locking down the Extensions folder, and block downloading the xpi files.

Best regards, Sea-you

You can lock the pref xpinstall.enabled to false

lockPref("xpinstall.enabled" ,false);

See also http://kb.mozillazine.org/Locking_preferences