There are some methods of locking things down. Many are out of the scope of this forum because you may also need to consider preventing users installing and using

• another copy of Firefox, or another new profile
• Other browsers
• Even installing a browser or complete OS maybe on removable media

If it is protecting minors maybe the service provider has an option to Filter content,or there is OS level software that can be considered. But remember all children have friends and many have smartphones etc.

Using Linux has pros and cons. Many users will be less familiar with it and less able to change things, but those that are familiar may well be more computer literate and more able to circumvent your restrictions.

If it is a business System and you are the Admin then you will probably not be looking at a Firefox solution.

See also

• http://kb.mozillazine.org/Locking_preferences
• https://addons.mozilla.org/firefox/addon/cck/
  • and a particular developer's series of blogs e.g. http://mike.kaply.com/2013/09/25/getting-ready-for-cck2/

I am using Ubuntu. The Os-Level an ISP level content filtering solutions can easily be bypassed using https proxies. So I am looking for a browser based solution. I was wondering if a firefox extension can be made impossible to remove ( for non-admins). CCK says it does that. Then do I have to install CCK as root ? Is it impossible to disable it for unprevilaged users.

You can lock network prefs via a mozilla.cfg file in the installation directory to prevent other users from changing the connection settings if that is all you want to achieve to make sure that a specific proxy is used.

Use a mozilla.cfg file in the Firefox program folder to lock prefs or specify new (default) values.

Place a file local-settings.js in the defaults\pref folder where you also find the file channel-prefs.js to specify using mozilla.cfg.

pref("general.config.filename", "mozilla.cfg");

See:

• http://kb.mozillazine.org/Locking_preferences

These functions can be used in the mozilla.cfg file:

defaultPref();  // set new default value
pref();         // set pref, but allow changes in current session
lockPref();     // lock pref, disallow changes

• http://kb.mozillazine.org/network.proxy.type

This post here by Michael Kaply (the author of CCK) provides a solution.